diff --git a/fluent-plugin-fluent-package-update-notifier.gemspec b/fluent-plugin-fluent-package-update-notifier.gemspec
index d6c36cd..d5a4e97 100644
--- a/fluent-plugin-fluent-package-update-notifier.gemspec
+++ b/fluent-plugin-fluent-package-update-notifier.gemspec
@@ -32,7 +32,7 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler", "~> 2.4"
   spec.add_development_dependency "rake", "~> 13.2.1"
-  spec.add_development_dependency "test-unit", "~> 3.6.7"
+  spec.add_development_dependency "test-unit", "~> 3.0"
   spec.add_development_dependency "test-unit-rr", "~> 1.0.5"
   spec.add_runtime_dependency "fluentd", [">= 0.14.10", "< 2"]