diff --git a/v2_0_cpp_variant.md b/v2_0_cpp_variant.md
index 0aaf4bb..b7c38e8 100644
--- a/v2_0_cpp_variant.md
+++ b/v2_0_cpp_variant.md
@@ -2,7 +2,7 @@
 
 ## What is this?
 
-msgpack-c doesn't have `boost::variant` adaptor. But `msgpack::type::variant` can contain any msgpack types. It is implemented using `boost::variant`. There are two variant types. One is value based `msgpack::type::variant`, the other is reference based `msgpack::type::variant_ref`.
+msgpack-c doesn't have `boost::variant` adaptor. But `msgpack::type::variant` can contain any msgpack types. It is implemented using `boost::variant`. There are two variant types. One is value based `msgpack::type::variant`, the other is reference based `msgpack::type::variant_ref`. When you use `msgpack::type::variant`, you need to define [MSGPACK_USE_BOOST](../v2_0_cpp_configure#msgpack_use_boost-since-120) macro.
  
 ## Type mapping