diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9c563a2..450bfd7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ jobs:
     strategy:
       matrix:
         os:   ['ubuntu', 'macos']
-        ruby: [ '2.6', '2.7', '3.0' ]
+        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0' ]
     steps:
       - uses: actions/checkout@v2
       - uses: ruby/setup-ruby@v1