diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index f8cd43b..0e57388 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -1,6 +1,7 @@
 name: build
 on:
-  push: { branches: [main] }
+  push:
+    branches: [main]
   release:
     types: [published]
 jobs:
@@ -10,6 +11,7 @@ jobs:
       - uses: actions/checkout@v2
       - run: sudo apt-get update
       - run: sudo apt-get install -y qemu-user-static
+      - run: sudo bin/overlay-gen
       - run: bin/build
       - run: sudo bin/img-gen
       - uses: actions/upload-artifact@v2
diff --git a/.gitignore b/.gitignore
index 96bf745..2d69006 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 /apkovl/
 /cache/
 /dist/
+/overlay.img.gz
 /yama-armhf.tar.gz
 /yama-armhf.img.gz
diff --git a/README.md b/README.md
index c6d4467..cfee0f3 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,7 @@ Raspberry Pi Zero 向け OS のビルドするためのツール群
 ## ビルド
 
 ```sh
+bin/overlay-gen
 bin/build
 bin/img-gen
 ```
@@ -20,7 +21,7 @@ bin/img-gen
 - Docker
 - Docker Compose
 - qemu-user-static
-- awk, bash, curl, fallocate, grep, gzip, losetup, mkfs.fat, parted, python3, ssh-keygen, tar, xargs
+- awk, bash, curl, fallocate, grep, gzip, losetup, mkfs.ext4, mkfs.fat, parted, python3, ssh-keygen, tar, xargs
 
 ### WiFi の設定
 
diff --git a/bin/overlay-gen b/bin/overlay-gen
new file mode 100755
index 0000000..6ea1854
--- /dev/null
+++ b/bin/overlay-gen
@@ -0,0 +1,20 @@
+#!/bin/sh
+# オーバーレイイメージファイルの生成
+# Usage: bin/overlay-gen
+# Required: fallocate losetup mkfs.ext4 gzip
+cd -- "$(dirname -- "$0")/.."
+set -eux
+size=512m
+fallocate -l "${size}" overlay.img
+mkfs.ext4 overlay.img
+dev="$(losetup -f)"
+finally="losetup -d '${dev}'"
+trap 'sh -c "${finally}"' EXIT
+losetup "${dev}" overlay.img
+tmp="$(mktemp -d)"
+finally="rmdir '${tmp}'; ${finally}"
+mount "${dev}" "${tmp}"
+finally="umount '${tmp}'; ${finally}"
+mkdir -p "${tmp}/work/usr"
+mkdir -p "${tmp}/upper/usr"
+finally="${finally}; gzip -f overlay.img"
diff --git a/overlay.img.gz b/overlay.img.gz
deleted file mode 100644
index 84846ef..0000000
Binary files a/overlay.img.gz and /dev/null differ