diff --git a/README.md b/README.md
index 88faa6b..d1a781b 100644
--- a/README.md
+++ b/README.md
@@ -149,6 +149,7 @@ flowchart LR
 
 Each processing step corresponds to a dedicated GUI widget,  
 and all results (images, contours, metadata, EFDs) are automatically exported to the `output/` directory.
+For standalone builds, `output/` is created next to the app/executable when writable; otherwise it falls back to the OS user data directory.
 
 For a detailed step-by-step guide, please refer to the [Usage page](https://maple60.github.io/leaf-contour-efd/usage.html).
 
diff --git a/README_ja.md b/README_ja.md
index 8996a67..bf5f7f7 100644
--- a/README_ja.md
+++ b/README_ja.md
@@ -147,6 +147,7 @@ flowchart LR
 
 各処理ステップは専用のGUIウィジェットに対応しており、  
 解析結果（画像・輪郭・メタデータ・EFD）は自動的に `output/` ディレクトリに保存されます。
+スタンドアロン版では、書き込み可能な場合はアプリ/実行ファイルと同じ場所に `output/` を作成し、不可の場合はOSのユーザーデータ領域へ自動的にフォールバックします。
 
 詳しい操作手順については、[Usage ページ](https://maple60.github.io/leaf-contour-efd/usage.html) を参照してください。
 
diff --git a/src/leaf_contour_efd/utils/paths.py b/src/leaf_contour_efd/utils/paths.py
index 370d8f5..5eee579 100644
--- a/src/leaf_contour_efd/utils/paths.py
+++ b/src/leaf_contour_efd/utils/paths.py
@@ -28,17 +28,34 @@ def _is_writable_dir(path: Path) -> bool:
         return False
 
 
+def _frozen_portable_base_dir() -> Path | None:
+    """Return a portable base dir near the frozen app/executable when possible."""
+    executable = Path(sys.executable).resolve()
+
+    if sys.platform == "darwin":
+        # pyinstaller .app executable path example:
+        # /path/MyApp.app/Contents/MacOS/MyApp
+        app_bundle = next((p for p in executable.parents if p.suffix == ".app"), None)
+        if app_bundle is not None:
+            return app_bundle.parent
+
+    return executable.parent
+
+
 def get_output_base_dir() -> Path:
     """Return the base directory used for writing the ``output`` folder.
 
     Rules
     -----
-    - Frozen on all platforms: user-writable application data directory.
+    - Frozen: directory near app/executable when writable, otherwise user data.
     - Non-frozen: repository/application root when writable, otherwise user data.
     """
     fallback = _user_writable_base_dir()
 
     if getattr(sys, "frozen", False):
+        portable_base = _frozen_portable_base_dir()
+        if portable_base is not None and _is_writable_dir(portable_base):
+            return portable_base
         return fallback
 
     # Development mode: src/leaf_contour_efd/utils/paths.py -> repo root