diff --git a/src/file.c b/src/file.c
index 24a95f49a..b43f092fa 100644
--- a/src/file.c
+++ b/src/file.c
@@ -4,6 +4,7 @@
 
 #include "mruby.h"
 
+#include "mruby/ext/io.h"
 #include "mruby/class.h"
 #include "mruby/data.h"
 #include "mruby/string.h"