diff --git a/test/plugin/test_out_buffered_stdout.rb b/test/plugin/test_out_buffered_stdout.rb
index <HASH>..<HASH> 100644
--- a/test/plugin/test_out_buffered_stdout.rb
+++ b/test/plugin/test_out_buffered_stdout.rb
@@ -1,5 +1,6 @@
 require_relative '../helper'
 require 'fluent/test'
+require 'fluent/plugin/out_buffered_stdout'
 
 class BufferedStdoutOutputTest < Test::Unit::TestCase
   def setup
Add missing requires in buffered output plugin tests