diff --git a/ext/rbs_extension/extconf.rb b/ext/rbs_extension/extconf.rb
index 986b17f3f..5acc0a640 100644
--- a/ext/rbs_extension/extconf.rb
+++ b/ext/rbs_extension/extconf.rb
@@ -1,4 +1,4 @@
 require 'mkmf'
 $INCFLAGS << " -I$(top_srcdir)" if $extmk
-$CFLAGS += " -std=c99 -Wold-style-definition"
+$CFLAGS += " -std=c99 -Wold-style-definition" unless have_macro('_MSC_VER')
 create_makefile 'rbs_extension'