diff --git a/compile_flag.cmake b/compile_flag.cmake
index 455863b..0f78b54 100644
--- a/compile_flag.cmake
+++ b/compile_flag.cmake
@@ -1,3 +1,5 @@
+include(CheckCXXCompilerFlag)
+
 macro (append_compile_flag _flag)
     string (MAKE_C_IDENTIFIER ${_flag} _name)