diff --git a/lib/fluent/config.rb b/lib/fluent/config.rb
index b2411853e1..7ccbf23552 100644
--- a/lib/fluent/config.rb
+++ b/lib/fluent/config.rb
@@ -62,6 +62,7 @@ def self.parse(str, fname, basepath = Dir.pwd, v1_config = nil, syntax: :v1)
         Parser.parse(str, fname, basepath)
       when :ruby
         require 'fluent/config/dsl'
+        $log.warn("Ruby DSL configuration format is deprecated. Please use original configuration format. https://docs.fluentd.org/configuration/config-file")
         Config::DSL::Parser.parse(str, File.join(basepath, fname))
       else
         raise "[BUG] unknown configuration parser specification:'#{parser}'"