diff --git a/docs/doxygen_idlref.conf.cmake.in b/docs/doxygen_idlref.conf.cmake.in
index 0699c1b9d..98f779a11 100644
--- a/docs/doxygen_idlref.conf.cmake.in
+++ b/docs/doxygen_idlref.conf.cmake.in
@@ -38,7 +38,7 @@ PROJECT_NAME           = "OpenRTM-aist IDL"
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = @RTM_VERSION@
+PROJECT_NUMBER         = @RTM_SO_VERSION@
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a