diff --git a/fluentd.gemspec b/fluentd.gemspec
index cc66216f03..bef8f20fe2 100644
--- a/fluentd.gemspec
+++ b/fluentd.gemspec
@@ -51,5 +51,6 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency("test-unit", ["~> 3.3"])
   gem.add_development_dependency("test-unit-rr", ["~> 1.0"])
   gem.add_development_dependency("oj", [">= 2.14", "< 4"])
+  gem.add_development_dependency("async", "~> 1.23")
   gem.add_development_dependency("async-http", ">= 0.50.0")
 end