diff --git a/.github/workflows/gh-pages.yml b/.github/workflows/gh-pages.yml
index f250e4b..0f76439 100644
--- a/.github/workflows/gh-pages.yml
+++ b/.github/workflows/gh-pages.yml
@@ -19,7 +19,7 @@ env:
   LANG: 'ja_JP.UTF-8'
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2