diff --git a/ChangeLog b/ChangeLog
index ecec958b62..78dd070c78 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun May  8 08:19:16 2016  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* configure.in: add -Werror=implicit-int to avoid missing type of
+	  function declaration.
+
 Sat May  7 22:22:37 2016  SHIBATA Hiroshi  <hsbt@ruby-lang.org>
 
 	* lib/webrick/ssl.rb: Accept string value for SSLCertName. It is used
diff --git a/configure.in b/configure.in
index d24619e9a0..319f3f8a6d 100644
--- a/configure.in
+++ b/configure.in
@@ -826,6 +826,7 @@ if test "$GCC:${warnflags+set}:no" = yes::no; then
 		 -Wno-constant-logical-operand \
 		 -Wno-self-assign \
 		 -Wunused-variable \
+		 -Werror=implicit-int \
 		 -Werror=pointer-arith \
 		 -Werror=write-strings \
 		 -Werror=declaration-after-statement \