diff --git a/cpp/configure.in b/cpp/configure.in
index b05f9183..0cfc43ff 100644
--- a/cpp/configure.in
+++ b/cpp/configure.in
@@ -4,10 +4,10 @@ AM_INIT_AUTOMAKE(msgpack, 0.5.6)
 AC_CONFIG_HEADER(config.h)
 
 AC_SUBST(CFLAGS)
-CFLAGS="-O4 -Wall $CFLAGS"
+CFLAGS="-O3 -Wall $CFLAGS"
 
 AC_SUBST(CXXFLAGS)
-CXXFLAGS="-O4 -Wall $CXXFLAGS"
+CXXFLAGS="-O3 -Wall $CXXFLAGS"
 
 
 AC_PROG_CC