diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2b5656e2..e74dd5a7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -11,6 +11,8 @@ project("SoftEther VPN"
   LANGUAGES C
 )
 
+set(CMAKE_C_STANDARD 99)
+
 set(TOP_DIRECTORY ${CMAKE_SOURCE_DIR})
 set(BUILD_DIRECTORY ${CMAKE_BINARY_DIR})