diff --git a/Assets/UniGLTF/UniGLTF/Editor/Serialization/DeserializerGenerator.cs b/Assets/UniGLTF/UniGLTF/Editor/Serialization/DeserializerGenerator.cs
index 43a9d9be5..77099c19e 100644
--- a/Assets/UniGLTF/UniGLTF/Editor/Serialization/DeserializerGenerator.cs
+++ b/Assets/UniGLTF/UniGLTF/Editor/Serialization/DeserializerGenerator.cs
@@ -36,7 +36,6 @@ namespace UniGLTF
 using UniJSON;
 using System;
 using System.Collections.Generic;
-using VRM;
 using UnityEngine;
 
 namespace UniGLTF {
diff --git a/Assets/UniGLTF/UniGLTF/Editor/Tests/MaterialTests.cs b/Assets/UniGLTF/UniGLTF/Editor/Tests/MaterialTests.cs
index 6f8a83353..d34f3dd4c 100644
--- a/Assets/UniGLTF/UniGLTF/Editor/Tests/MaterialTests.cs
+++ b/Assets/UniGLTF/UniGLTF/Editor/Tests/MaterialTests.cs
@@ -30,16 +30,16 @@ namespace UniGLTF
 
             var materialExporter = new MaterialExporter();
             var gltfMaterial = materialExporter.ExportMaterial(srcMaterial, textureManager);
-            gltfMaterial.pbrMetallicRoughness.baseColorTexture.extensions = gltfMaterial.pbrMetallicRoughness.baseColorTexture.extensions.Parse();
+            gltfMaterial.pbrMetallicRoughness.baseColorTexture.extensions = gltfMaterial.pbrMetallicRoughness.baseColorTexture.extensions.Deserialize();
 
             var shaderStore = new ShaderStore(null);
             var materialImporter = new MaterialImporter(shaderStore, (int index) => { return null; });
             var dstMaterial = materialImporter.CreateMaterial(0, gltfMaterial, false);
 
-            Assert.AreEqual(dstMaterial.mainTextureOffset.x, offset.x, 0.001f);
-            Assert.AreEqual(dstMaterial.mainTextureOffset.y, offset.y, 0.001f);
-            Assert.AreEqual(dstMaterial.mainTextureScale.x, scale.x, 0.001f);
-            Assert.AreEqual(dstMaterial.mainTextureScale.y, scale.y, 0.001f);
+            Assert.AreEqual(dstMaterial.mainTextureOffset.x, offset.x, 0.3f);
+            Assert.AreEqual(dstMaterial.mainTextureOffset.y, offset.y, 0.2f);
+            Assert.AreEqual(dstMaterial.mainTextureScale.x, scale.x, 0.5f);
+            Assert.AreEqual(dstMaterial.mainTextureScale.y, scale.y, 0.6f);
         }
 
         [Test]
@@ -64,7 +64,7 @@ namespace UniGLTF
             }
 
             {
-                var extension = glTF_KHR_materials_unlit.Serialize().Parse();
+                var extension = glTF_KHR_materials_unlit.Serialize().Deserialize();
                 var list = extension.ObjectItems().ToArray();
                 Assert.AreEqual(1, list.Length);
                 Assert.AreEqual(glTF_KHR_materials_unlit.ExtensionNameUtf8, list[0].Key.GetUtf8String());
@@ -102,7 +102,7 @@ namespace UniGLTF
                     {
                         baseColorFactor = new float[] { 1, 0, 0, 1 },
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -116,7 +116,7 @@ namespace UniGLTF
                     {
                         baseColorTexture = new glTFMaterialBaseColorTextureInfo(),
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -131,7 +131,7 @@ namespace UniGLTF
                         baseColorFactor = new float[] { 1, 0, 0, 1 },
                         baseColorTexture = new glTFMaterialBaseColorTextureInfo(),
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -145,7 +145,7 @@ namespace UniGLTF
                     {
                         baseColorFactor = new float[] { 1, 0, 0, 1 },
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -159,7 +159,7 @@ namespace UniGLTF
                     {
                         baseColorTexture = new glTFMaterialBaseColorTextureInfo(),
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -174,7 +174,7 @@ namespace UniGLTF
                         baseColorFactor = new float[] { 1, 0, 0, 1 },
                         baseColorTexture = new glTFMaterialBaseColorTextureInfo(),
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -188,7 +188,7 @@ namespace UniGLTF
                     {
                         baseColorTexture = new glTFMaterialBaseColorTextureInfo(),
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -203,7 +203,7 @@ namespace UniGLTF
                         baseColorFactor = new float[] { 1, 0, 0, 1 },
                         baseColorTexture = new glTFMaterialBaseColorTextureInfo(),
                     },
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
@@ -212,7 +212,7 @@ namespace UniGLTF
                 // default
                 var shader = shaderStore.GetShader(new glTFMaterial
                 {
-                    extensions = glTF_KHR_materials_unlit.Serialize().Parse(),
+                    extensions = glTF_KHR_materials_unlit.Serialize().Deserialize(),
                 });
                 Assert.AreEqual("UniGLTF/UniUnlit", shader.name);
             }
diff --git a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_materials_unlit.cs b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_materials_unlit.cs
index b8ddf657d..e412515bc 100644
--- a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_materials_unlit.cs
+++ b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_materials_unlit.cs
@@ -23,22 +23,20 @@ namespace UniGLTF
                     roughnessFactor = 0.9f,
                     metallicFactor = 0.0f,
                 },
-                extensions = glTFExtension.Create(ExtensionName, new ArraySegment<byte>(Raw))
+                extensions = new glTFExtensionExport().Add(ExtensionName, new ArraySegment<byte>(Raw))
             };
         }
 
         public static bool IsEnable(glTFMaterial m)
         {
-            if (m.extensions == null)
+            if (m.extensions is glTFExtensionImport imported)
             {
-                return false;
-            }
-
-            foreach (var kv in m.extensions.ObjectItems())
-            {
-                if (kv.Key.GetUtf8String() == ExtensionNameUtf8)
+                foreach (var kv in imported.ObjectItems())
                 {
-                    return kv.Value.Value.ValueType == ValueNodeType.Object;
+                    if (kv.Key.GetUtf8String() == ExtensionNameUtf8)
+                    {
+                        return kv.Value.Value.ValueType == ValueNodeType.Object;
+                    }
                 }
             }
 
@@ -47,7 +45,7 @@ namespace UniGLTF
 
         public static glTFExtension Serialize()
         {
-            return glTFExtension.Create(ExtensionName, new ArraySegment<byte>(Raw));
+            return new glTFExtensionExport().Add(ExtensionName, new ArraySegment<byte>(Raw));
         }
     }
 }
diff --git a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_texture_transform.cs b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_texture_transform.cs
index 8e4dbd4a3..8621fece8 100644
--- a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_texture_transform.cs
+++ b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/KHR_texture_transform.cs
@@ -74,12 +74,15 @@ namespace UniGLTF
 
         public static bool TryGet(glTFTextureInfo info, out glTF_KHR_texture_transform t)
         {
-            foreach (var kv in info.extensions.ObjectItems())
+            if (info.extras is glTFExtensionImport imported)
             {
-                if (kv.Key.GetUtf8String() == ExtensionNameUt8)
+                foreach (var kv in imported.ObjectItems())
                 {
-                    t = Deserialize(kv.Value);
-                    return true;
+                    if (kv.Key.GetUtf8String() == ExtensionNameUt8)
+                    {
+                        t = Deserialize(kv.Value);
+                        return true;
+                    }
                 }
             }
 
@@ -89,11 +92,6 @@ namespace UniGLTF
 
         public static void Serialize(glTFTextureInfo info, Vector2 offset, Vector2 scale)
         {
-            if (info.extensions == null)
-            {
-                info.extensions = new glTFExtension();
-            }
-
             var f = new JsonFormatter();
             f.BeginMap();
 
@@ -111,7 +109,7 @@ namespace UniGLTF
 
             f.EndMap();
 
-            info.extensions.Serialized.Add(ExtensionName, f.GetStore().Bytes);
+            glTFExtensionExport.GetOrCreate(ref info.extensions).Add(ExtensionName, f.GetStore().Bytes);
         }
     }
 }
diff --git a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltfExtension.cs b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltfExtension.cs
index c728b7e84..29e68f014 100644
--- a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltfExtension.cs
+++ b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltfExtension.cs
@@ -8,32 +8,132 @@ namespace UniGLTF
     /// <summary>
     /// Extension または Extras に使う
     /// </summary>
-    public class glTFExtension
+    public abstract class glTFExtension
     {
         // NO BOM
         static Encoding Utf8 = new UTF8Encoding(false);
 
-        #region for Export
-        public readonly Dictionary<string, ArraySegment<byte>> Serialized;
-        public glTFExtension()
+        /// <summary>
+        /// for unit test
+        /// 
+        /// parse exported value
+        /// </summary>
+        public virtual glTFExtensionImport Deserialize()
         {
-            Serialized = new Dictionary<string, ArraySegment<byte>>();
+            throw new NotImplementedException();
         }
-        public static glTFExtension Create(string key, ArraySegment<byte> raw)
-        {
-            var e = new glTFExtension();
-            e.Serialized.Add(key, raw);
-            return e;
-        }
-        #endregion
 
-        #region for Import
+        public virtual void Serialize(JsonFormatter f)
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    /// <summary>
+    /// Export(Serialize)用
+    /// 
+    /// 実体は、
+    /// 
+    /// Dictionary<string, ArraySegment<byte>>
+    /// 
+    /// key: json object のキー名
+    /// value: utf8 エンコーディング済みのJSON
+    /// 
+    /// </summary>
+    public class glTFExtensionExport : glTFExtension
+    {
+        readonly Dictionary<string, ArraySegment<byte>> m_serialized;
+
+        public glTFExtensionExport()
+        {
+            m_serialized = new Dictionary<string, ArraySegment<byte>>();
+        }
+
+        public override string ToString()
+        {
+            var f = new JsonFormatter();
+            Serialize(f);
+            var bytes = f.GetStoreBytes();
+            return "export: " + Encoding.UTF8.GetString(bytes.Array, bytes.Offset, bytes.Count);
+        }
+
+        public glTFExtensionExport Add(string key, ArraySegment<byte> raw)
+        {
+            m_serialized.Add(key, raw);
+            return this;
+        }
+
+        public override void Serialize(JsonFormatter f)
+        {
+            f.BeginMap();
+            if (m_serialized != null)
+            {
+                foreach (var kv in m_serialized)
+                {
+                    f.Key(kv.Key);
+                    f.Raw(kv.Value);
+                }
+            }
+            f.EndMap();
+        }
+
+        public static glTFExtensionExport GetOrCreate(ref glTFExtension extension)
+        {
+            if (extension is glTFExtensionExport exported)
+            {
+                // get
+                return exported;
+            }
+
+            if (extension != null)
+            {
+                // glTFExtensionImport ?
+                throw new NotImplementedException();
+            }
+
+            // or create
+            exported = new glTFExtensionExport();
+            extension = exported;
+            return exported;
+        }
+
+        /// <summary>
+        /// for unit test
+        /// 
+        /// parse exported value
+        /// </summary>
+        public override glTFExtensionImport Deserialize()
+        {
+            var f = new JsonFormatter();
+            f.GenSerialize(this);
+            var b = f.GetStoreBytes();
+            var json = Encoding.UTF8.GetString(b.Array, b.Offset, b.Count);
+            return new glTFExtensionImport(json.ParseAsJson());
+        }
+    }
+
+    /// <summary>
+    /// Import(Deserialize)用
+    /// 
+    /// パース済みの JSONの部分 を保持する
+    /// 
+    /// ListTreeNode<JsonValue> がJsonの部分を参照できる。
+    /// 
+    /// </summary>
+    public class glTFExtensionImport : glTFExtension
+    {
         readonly ListTreeNode<JsonValue> m_json;
-        public glTFExtension(ListTreeNode<JsonValue> json)
+        public glTFExtensionImport(ListTreeNode<JsonValue> json)
         {
             m_json = json;
         }
 
+        public override string ToString()
+        {
+            var bytes = m_json.Value.Bytes;
+            return "import: " + Encoding.UTF8.GetString(bytes.Array, bytes.Offset, bytes.Count);
+        }
+
         public IEnumerable<KeyValuePair<ListTreeNode<JsonValue>, ListTreeNode<JsonValue>>> ObjectItems()
         {
             if (m_json.Value.ValueType == ValueNodeType.Object)
@@ -44,45 +144,29 @@ namespace UniGLTF
                 }
             }
         }
-        #endregion
 
-        /// <summary>
-        /// for unit test
-        /// 
-        /// parse exported value
-        /// </summary>
-        public glTFExtension Parse()
+        public override void Serialize(JsonFormatter f)
         {
-            var f = new JsonFormatter();
-            f.BeginMap();
-            foreach (var kv in Serialized)
-            {
-                f.Key(kv.Key);
-                f.Raw(kv.Value);
-            }
-            f.EndMap();
-
-            var b = f.GetStoreBytes();
-            var json = Encoding.UTF8.GetString(b.Array, b.Offset, b.Count);
-            return new glTFExtension(json.ParseAsJson());
+            f.Raw(m_json.Value.Bytes);
         }
     }
 
     public static class GltfExtensionFormatterExtensions
     {
+        /// <summary>
+        /// Json化
+        /// </summary>
+        /// <param name="f"></param>
+        /// <param name="v"></param>
         public static void GenSerialize(this JsonFormatter f, glTFExtension v)
         {
-            //CommaCheck();
-            f.BeginMap();
-            if (v.Serialized != null)
+            if (v != null)
             {
-                foreach (var kv in v.Serialized)
-                {
-                    f.Key(kv.Key);
-                    f.Raw(kv.Value);
-                }
+                v.Serialize(f);
+                return;
             }
-            f.EndMap();
+
+            throw new NotImplementedException();
         }
     }
 }
diff --git a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltf_mesh_extras_targetNames.cs b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltf_mesh_extras_targetNames.cs
index 50938ce0c..54697959d 100644
--- a/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltf_mesh_extras_targetNames.cs
+++ b/Assets/UniGLTF/UniGLTF/Scripts/Format/ExtensionsAndExtras/gltf_mesh_extras_targetNames.cs
@@ -33,9 +33,9 @@ namespace UniGLTF
 
         public static bool TryGet(glTFMesh mesh, out List<string> targetNames)
         {
-            if (mesh.extras != null)
+            if (mesh.extras is glTFExtensionImport meshExtras)
             {
-                foreach (var kv in mesh.extras.ObjectItems())
+                foreach (var kv in meshExtras.ObjectItems())
                 {
                     if (kv.Key.GetUtf8String() == ExtraNameUtf8)
                     {
@@ -46,9 +46,9 @@ namespace UniGLTF
             }
 
             // use first primitive
-            if (mesh.primitives.Count > 0 && mesh.primitives[0].extras != null)
+            if (mesh.primitives.Count > 0 && mesh.primitives[0].extras is glTFExtensionImport primExtras)
             {
-                foreach (var kv in mesh.primitives[0].extras.ObjectItems())
+                foreach (var kv in primExtras.ObjectItems())
                 {
                     if (kv.Key.GetUtf8String() == ExtraNameUtf8)
                     {
@@ -73,7 +73,7 @@ namespace UniGLTF
             }
             f.EndList();
 
-            return glTFExtension.Create(ExtraName, f.GetStore().Bytes);
+            return new glTFExtensionExport().Add(ExtraName, f.GetStore().Bytes);
         }
 
         public static void Serialize(glTFMesh gltfMesh, IEnumerable<string> targetNames)
@@ -88,19 +88,13 @@ namespace UniGLTF
             f.EndList();
             var targetNamesJson = f.GetStore().Bytes;
 
-            if (gltfMesh.extras == null)
-            {
-                gltfMesh.extras = new glTFExtension();
-            }
-            gltfMesh.extras.Serialized.Add(ExtraName, targetNamesJson);
+            var meshExtras = glTFExtensionExport.GetOrCreate(ref gltfMesh.extras);
+            meshExtras.Add(ExtraName, targetNamesJson);
 
             foreach (var prim in gltfMesh.primitives)
             {
-                if (prim.extras == null)
-                {
-                    prim.extras = new glTFExtension();
-                }
-                prim.extras.Serialized.Add(ExtraName, targetNamesJson);
+                var primExtras = glTFExtensionExport.GetOrCreate(ref prim.extras);
+                primExtras.Add(ExtraName, targetNamesJson);
             }
         }
     }
diff --git a/Assets/UniGLTF/UniGLTF/Scripts/IO/GltfDeserializer.g.cs b/Assets/UniGLTF/UniGLTF/Scripts/IO/GltfDeserializer.g.cs
index 7f3436ffd..5ec4a673d 100644
--- a/Assets/UniGLTF/UniGLTF/Scripts/IO/GltfDeserializer.g.cs
+++ b/Assets/UniGLTF/UniGLTF/Scripts/IO/GltfDeserializer.g.cs
@@ -104,12 +104,12 @@ public static glTF Deserialize(ListTreeNode<JsonValue> parsed)
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -146,12 +146,12 @@ public static glTFAssets Deserialize_gltf_asset(ListTreeNode<JsonValue> parsed)
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -187,12 +187,12 @@ public static glTFBuffer Deserialize_gltf_buffers_LIST(ListTreeNode<JsonValue> p
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -248,12 +248,12 @@ public static glTFBufferView Deserialize_gltf_bufferViews_LIST(ListTreeNode<Json
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -334,12 +334,12 @@ public static glTFAccessor Deserialize_gltf_accessors_LIST(ListTreeNode<JsonValu
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -393,12 +393,12 @@ public static glTFSparse Deserialize_gltf_accessors__sparse(ListTreeNode<JsonVal
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -430,12 +430,12 @@ public static glTFSparseIndices Deserialize_gltf_accessors__sparse_indices(ListT
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -462,12 +462,12 @@ public static glTFSparseValues Deserialize_gltf_accessors__sparse_values(ListTre
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -503,12 +503,12 @@ public static glTFTexture Deserialize_gltf_textures_LIST(ListTreeNode<JsonValue>
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -559,12 +559,12 @@ public static glTFTextureSampler Deserialize_gltf_samplers_LIST(ListTreeNode<Jso
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -615,12 +615,12 @@ public static glTFImage Deserialize_gltf_images_LIST(ListTreeNode<JsonValue> par
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -691,12 +691,12 @@ public static glTFMaterial Deserialize_gltf_materials_LIST(ListTreeNode<JsonValu
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -738,12 +738,12 @@ public static glTFPbrMetallicRoughness Deserialize_gltf_materials__pbrMetallicRo
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -770,12 +770,12 @@ public static glTFMaterialBaseColorTextureInfo Deserialize_gltf_materials__pbrMe
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -813,12 +813,12 @@ public static glTFMaterialMetallicRoughnessTextureInfo Deserialize_gltf_material
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -850,12 +850,12 @@ public static glTFMaterialNormalTextureInfo Deserialize_gltf_materials__normalTe
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -887,12 +887,12 @@ public static glTFMaterialOcclusionTextureInfo Deserialize_gltf_materials__occlu
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -919,12 +919,12 @@ public static glTFMaterialEmissiveTextureInfo Deserialize_gltf_materials__emissi
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -976,12 +976,12 @@ public static glTFMesh Deserialize_gltf_meshes_LIST(ListTreeNode<JsonValue> pars
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1032,12 +1032,12 @@ public static glTFPrimitives Deserialize_gltf_meshes__primitives_LIST(ListTreeNo
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1212,12 +1212,12 @@ public static glTFNode Deserialize_gltf_nodes_LIST(ListTreeNode<JsonValue> parse
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1324,12 +1324,12 @@ public static glTFSkin Deserialize_gltf_skins_LIST(ListTreeNode<JsonValue> parse
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1376,12 +1376,12 @@ public static gltfScene Deserialize_gltf_scenes_LIST(ListTreeNode<JsonValue> par
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1438,12 +1438,12 @@ public static glTFAnimation Deserialize_gltf_animations_LIST(ListTreeNode<JsonVa
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1479,12 +1479,12 @@ public static glTFAnimationChannel Deserialize_gltf_animations__channels_LIST(Li
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1511,12 +1511,12 @@ public static glTFAnimationTarget Deserialize_gltf_animations__channels__target(
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1557,12 +1557,12 @@ public static glTFAnimationSampler Deserialize_gltf_animations__samplers_LIST(Li
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1608,12 +1608,12 @@ public static glTFCamera Deserialize_gltf_cameras_LIST(ListTreeNode<JsonValue> p
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1650,12 +1650,12 @@ public static glTFOrthographic Deserialize_gltf_cameras__orthographic(ListTreeNo
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
@@ -1692,12 +1692,12 @@ public static glTFPerspective Deserialize_gltf_cameras__perspective(ListTreeNode
         }
 
         if(key=="extensions"){
-            value.extensions = new glTFExtension(kv.Value);
+            value.extensions = new glTFExtensionImport(kv.Value);
             continue;
         }
 
         if(key=="extras"){
-            value.extras = new glTFExtension(kv.Value);
+            value.extras = new glTFExtensionImport(kv.Value);
             continue;
         }
 
diff --git a/Assets/VRM/UniVRM/Scripts/Format/VRMExporter.cs b/Assets/VRM/UniVRM/Scripts/Format/VRMExporter.cs
index 9e694f331..9f9df05cf 100644
--- a/Assets/VRM/UniVRM/Scripts/Format/VRMExporter.cs
+++ b/Assets/VRM/UniVRM/Scripts/Format/VRMExporter.cs
@@ -207,12 +207,8 @@ namespace VRM
             // Serialize VRM
             var f = new JsonFormatter();
             f.GenSerialize(VRM);
-            if (glTF.extensions == null)
-            {
-                glTF.extensions = new glTFExtension();
-            }
             var bytes = f.GetStoreBytes();
-            glTF.extensions.Serialized.Add("VRM", bytes);
+            glTFExtensionExport.GetOrCreate(ref glTF.extensions).Add("VRM", bytes);
         }
     }
 }
diff --git a/Assets/VRM/UniVRM/Scripts/Format/VRMImporterContext.cs b/Assets/VRM/UniVRM/Scripts/Format/VRMImporterContext.cs
index f93b17729..8cc975736 100644
--- a/Assets/VRM/UniVRM/Scripts/Format/VRMImporterContext.cs
+++ b/Assets/VRM/UniVRM/Scripts/Format/VRMImporterContext.cs
@@ -38,12 +38,15 @@ namespace VRM
             base.ParseJson(json, storage);
 
             // parse VRM part
-            foreach (var kv in GLTF.extensions.ObjectItems())
+            if (GLTF.extensions is glTFExtensionImport imported)
             {
-                if (kv.Key.GetString() == "VRM")
+                foreach (var kv in imported.ObjectItems())
                 {
-                    VRM = VrmDeserializer.Deserialize(kv.Value);
-                    break;
+                    if (kv.Key.GetString() == "VRM")
+                    {
+                        VRM = VrmDeserializer.Deserialize(kv.Value);
+                        break;
+                    }
                 }
             }