diff --git a/lib/fluent/agent.rb b/lib/fluent/agent.rb
index 66ccfa50cf..b1f4b43820 100644
--- a/lib/fluent/agent.rb
+++ b/lib/fluent/agent.rb
@@ -135,7 +135,8 @@ def add_match(type, pattern, conf)
       output.router = @event_router if output.respond_to?(:router=)
       output.configure(conf)
       @outputs << output
-      if output.respond_to?(:outputs) && (output.is_a?(Fluent::Plugin::MultiOutput) || output.is_a?(Fluent::MultiOutput))
+      if output.respond_to?(:outputs) && (output.respond_to?(:multi_output?) && output.multi_output? || output.is_a?(Fluent::MultiOutput))
+        # TODO: ruby 2.3 or later: replace `output.respond_to?(:multi_output?) && output.multi_output?` with output&.multi_output?
         @outputs.push(*output.outputs)
       end
       @event_router.add_rule(pattern, output)
diff --git a/lib/fluent/plugin/multi_output.rb b/lib/fluent/plugin/multi_output.rb
index 51a6ef625c..56fb498948 100644
--- a/lib/fluent/plugin/multi_output.rb
+++ b/lib/fluent/plugin/multi_output.rb
@@ -53,6 +53,10 @@ def initialize
         # @rollback_count = 0
       end
 
+      def multi_output?
+        true
+      end
+
       def configure(conf)
         super