diff --git a/fluentd.gemspec b/fluentd.gemspec
index 195cca94d2..ff225597d3 100644
--- a/fluentd.gemspec
+++ b/fluentd.gemspec
@@ -63,6 +63,7 @@ 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")
   gem.add_development_dependency("aws-sigv4", ["~> 1.8"])
   gem.add_development_dependency("aws-sdk-core", ["~> 3.191"])