diff --git a/doc/string.rb b/doc/string.rdoc
similarity index 99%
rename from doc/string.rb
rename to doc/string.rdoc
index 65bfa39272..c9fa0607f3 100644
--- a/doc/string.rb
+++ b/doc/string.rdoc
@@ -1,3 +1,5 @@
+# :markup: ruby
+
 class String
   # call-seq:
   #   String.new(string = '') -> new_string
diff --git a/doc/transcode.rb b/doc/transcode.rdoc
similarity index 99%
rename from doc/transcode.rb
rename to doc/transcode.rdoc
index 5a7c57719f..29787fe907 100644
--- a/doc/transcode.rb
+++ b/doc/transcode.rdoc
@@ -1,3 +1,5 @@
+# :markup: ruby
+
 class String
   # call-seq:
   #   encode(dst_encoding = Encoding.default_internal, **enc_opts) -> string