diff --git a/man/ruby.1 b/man/ruby.1
index 779167ac3e..6c9f5fb0d1 100644
--- a/man/ruby.1
+++ b/man/ruby.1
@@ -388,7 +388,7 @@ before executing script.
 .Pp
 .It Fl y
 .It Fl -yydebug
-DO NOT USE.
+This option is not guaranteed to be compatible.
 .Pp
 Turns on compiler debug mode.  Ruby will print a bunch of internal
 state messages during compilation.  Only specify this switch you are going to
diff --git a/ruby.c b/ruby.c
index b9cfcffb2e..729a5ad01d 100644
--- a/ruby.c
+++ b/ruby.c
@@ -306,6 +306,7 @@ usage(const char *name, int help, int highlight, int columns)
         M("--backtrace-limit=num",                  "", "limit the maximum length of backtrace"),
         M("--verbose",                              "", "turn on verbose mode and disable script from stdin"),
         M("--version",                              "", "print the version number, then exit"),
+        M("-y",                          ", --yydebug", "print log of parser. Backward compatibility is not guaranteed"),
         M("--help",			            "", "show this message, -h for short message"),
     };
     static const struct ruby_opt_message dumps[] = {