diff --git a/CHANGELOG.md b/CHANGELOG.md
index 64c9eac..6b87688 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 
+## [Unreleased]
+
+### Fixed
+
+* Fix implementation of `iterator#increment` in `csmmap` and `csmvector`.
+
+
 ## [4-r.1] - 2020-01-30
 
 ### Added
@@ -20,4 +27,5 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * Reformat code using Prettier and ESLint.
 
 
+[Unreleased]: https://github.com/Live2D/CubismWebFramework/compare/4-r.1...HEAD
 [4-r.1]: https://github.com/Live2D/CubismWebFramework/compare/ce2585a919ac6e99f64dd468933772c6f1abbcc7...4-r.1
diff --git a/src/type/csmmap.ts b/src/type/csmmap.ts
index 64a4200..31d26ca 100644
--- a/src/type/csmmap.ts
+++ b/src/type/csmmap.ts
@@ -272,9 +272,7 @@ export namespace Live2DCubismFramework {
      */
     public increment(): iterator<_KeyT, _ValT> {
       const iteold = new iterator<_KeyT, _ValT>(this._map, this._index++); // 古い値を保存
-      this._map = iteold._map;
-      this._index = iteold._index;
-      return this;
+      return iteold;
     }
 
     /**
diff --git a/src/type/csmvector.ts b/src/type/csmvector.ts
index 4e105a5..a0c3c17 100644
--- a/src/type/csmvector.ts
+++ b/src/type/csmvector.ts
@@ -303,10 +303,8 @@ export namespace Live2DCubismFramework {
      * 後置き++演算子
      */
     public increment(): iterator<T> {
-      const iteold = new iterator<T>(this._vector, this._index++);
-      this._vector = iteold._vector;
-      this._index = iteold._index;
-      return this;
+      const iteold = new iterator<T>(this._vector, this._index++); // 古い値を保存
+      return iteold;
     }
 
     /**
@@ -314,9 +312,7 @@ export namespace Live2DCubismFramework {
      */
     public decrement(): iterator<T> {
       const iteold = new iterator<T>(this._vector, this._index--); // 古い値を保存
-      this._vector = iteold._vector;
-      this._index = iteold._index;
-      return this;
+      return iteold;
     }
 
     /**