diff --git a/ruby/extconf.rb b/ruby/extconf.rb
index f1d44ec1..10a807e7 100644
--- a/ruby/extconf.rb
+++ b/ruby/extconf.rb
@@ -1,5 +1,5 @@
 require 'mkmf'
 require './version.rb'
-$CFLAGS << %[ -I.. -Wall -O4 -DMESSAGEPACK_VERSION=\\"#{MessagePack::VERSION}\\" -g]
+$CFLAGS << %[ -I.. -Wall -O3 -DMESSAGEPACK_VERSION=\\"#{MessagePack::VERSION}\\" -g]
 create_makefile('msgpack')