diff --git a/fluentd.gemspec b/fluentd.gemspec
index cd942d72c1..3153a43c26 100644
--- a/fluentd.gemspec
+++ b/fluentd.gemspec
@@ -27,6 +27,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency("tzinfo", [">= 1.0", "< 3.0"])
   gem.add_runtime_dependency("tzinfo-data", ["~> 1.0"])
   gem.add_runtime_dependency("strptime", [">= 0.2.2", "< 1.0.0"])
+  gem.add_runtime_dependency("ext_monitor", [">= 0.2.2", '>= 0.1.1', "< 0.2"])
 
   # build gem for a certain platform. see also Rakefile
   fake_platform = ENV['GEM_BUILD_FAKE_PLATFORM'].to_s