diff --git a/Assets/UniGLTF/Runtime/UniGLTF/IO/GltfJsonUtil.cs b/Assets/UniGLTF/Runtime/UniGLTF/IO/GltfJsonUtil.cs
index 5492e5490..341a5bab9 100644
--- a/Assets/UniGLTF/Runtime/UniGLTF/IO/GltfJsonUtil.cs
+++ b/Assets/UniGLTF/Runtime/UniGLTF/IO/GltfJsonUtil.cs
@@ -9,6 +9,13 @@ namespace UniGLTF
 {
     public static class GltfJsonUtil
     {
+        /// <summary>
+        /// JsonPath を 再帰的に列挙する
+        /// object[] の中身は int(array index) or string(object key)
+        /// </summary>
+        /// <param name="node"></param>
+        /// <param name="path"></param>
+        /// <returns></returns>
         public static IEnumerable<object[]> TraverseJsonPath(JsonNode node, List<object> path)
         {
             if (path == null)
@@ -50,6 +57,43 @@ namespace UniGLTF
             return $"\"{src}\"";
         }
 
+        /// <summary>
+        /// jsonPath が
+        /// 
+        /// [..., "extensions", "EXTENSION_NAME"]
+        /// 
+        /// で有る場合に EXTENSION_NAME を返す。
+        /// </summary>
+        /// <param name="jsonPath"></param>
+        /// <param name="extensionName"></param>
+        /// <returns></returns>
+        static bool TryGetExtensionName(object[] path, out string extensionName)
+        {
+            if (path.Length >= 2)
+            {
+                if (path[path.Length - 2] is string x)
+                {
+                    if (x == "extensions")
+                    {
+                        if (path[path.Length - 1] is string y)
+                        {
+                            extensionName = y;
+                            return true;
+                        }
+                        else
+                        {
+                            // ありえない。はず
+                            var join = string.Join(", ", path);
+                            Debug.LogWarning($"invalid json path: {join}");
+                        }
+                    }
+                }
+            }
+
+            extensionName = default;
+            return false;
+        }
+
         /// <summary>
         /// https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/schema/glTF.schema.json
         /// 
@@ -63,19 +107,15 @@ namespace UniGLTF
         /// <returns></returns>
         public static string FindUsedExtensionsAndUpdateJson(string src)
         {
-            var used = new HashSet<string>();
             var parsed = src.ParseAsJson();
+
+            // unique な extension 名を収集
+            var used = new HashSet<string>();
             foreach (var path in TraverseJsonPath(parsed, null))
             {
-                if (path.Length >= 2)
+                if (TryGetExtensionName(path, out string extensionName))
                 {
-                    if (path[path.Length - 2] is string x)
-                    {
-                        if (x == "extensions")
-                        {
-                            used.Add(path[path.Length - 1] as string);
-                        }
-                    }
+                    used.Add(extensionName);
                 }
             }