diff --git a/ChangeLog b/ChangeLog
index 99bc4caef2..773026956d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Jun 15 17:14:58 2010  WATANABE Hirofumi  <eban@ruby-lang.org>
+
+	* ext/fiddle/extconf.rb: De Morgan's laws.
+
 Tue Jun 15 12:09:00 2010  WATANABE Hirofumi  <eban@ruby-lang.org>
 
 	* ext/fiddle/extconf.rb: use pkg_config to find ffi.h.
diff --git a/ext/fiddle/extconf.rb b/ext/fiddle/extconf.rb
index e2e1ea6d16..c80a1f9a83 100644
--- a/ext/fiddle/extconf.rb
+++ b/ext/fiddle/extconf.rb
@@ -4,7 +4,7 @@ require 'mkmf'
 
 dir_config 'libffi'
 
-unless pkg_config("libffi") and have_header('ffi.h')
+unless pkg_config("libffi") or have_header('ffi.h')
   if have_header('ffi/ffi.h')
     $defs.push(format('-DUSE_HEADER_HACKS'))
   else