diff --git a/test/plugin/test_in_tail.rb b/test/plugin/test_in_tail.rb
index 881ccc765a..382d4601f7 100644
--- a/test/plugin/test_in_tail.rb
+++ b/test/plugin/test_in_tail.rb
@@ -6,6 +6,7 @@
 require 'net/http'
 require 'flexmock/test_unit'
 require 'timecop'
+require 'tmpdir'
 require 'securerandom'
 
 class TailInputTest < Test::Unit::TestCase