diff --git a/test/bn_c_test.hpp b/test/bn_c_test.hpp
index d3ffb38..06ed271 100644
--- a/test/bn_c_test.hpp
+++ b/test/bn_c_test.hpp
@@ -363,7 +363,7 @@ CYBOZU_TEST_AUTO(serialize)
 	char buf[1024];
 	size_t n;
 	size_t expectSize;
-	int ret;
+	size_t ret;
 	// Fr
 	expectSize = G1Size;
 	mclBnFr_setInt(&x1, -1);