diff --git a/lib/writeexcel/workbook.rb b/lib/writeexcel/workbook.rb
index <HASH>..<HASH> 100644
--- a/lib/writeexcel/workbook.rb
+++ b/lib/writeexcel/workbook.rb
@@ -11,7 +11,6 @@
 # original written in Perl by John McNamara
 # converted to Ruby by Hideo Nakamura, cxn03651@msj.biglobe.ne.jp
 #
-require 'digest/md5'
 require 'nkf'
 require 'writeexcel/biffwriter'
 require 'writeexcel/worksheet'
* delete unneccessary require file.