diff --git a/lib/fluent/plugin/output.rb b/lib/fluent/plugin/output.rb
index 65d25e7201..5b42bb6cf1 100644
--- a/lib/fluent/plugin/output.rb
+++ b/lib/fluent/plugin/output.rb
@@ -122,12 +122,17 @@ def format(tag, time, record)
         raise NotImplementedError, "BUG: output plugins MUST implement this method"
       end
 
-      def formatted_to_msgpack_binary
+      def formatted_to_msgpack_binary?
         # To indicate custom format method (#format) returns msgpack binary or not.
         # If #format returns msgpack binary, override this method to return true.
         false
       end
 
+      # Compatibility for existing plugins
+      def formatted_to_msgpack_binary
+        formatted_to_msgpack_binary?
+      end
+
       def prefer_buffered_processing
         # override this method to return false only when all of these are true:
         #  * plugin has both implementation for buffered and non-buffered methods
diff --git a/test/plugin/test_output_as_buffered.rb b/test/plugin/test_output_as_buffered.rb
index e8240706df..2f94d35a13 100644
--- a/test/plugin/test_output_as_buffered.rb
+++ b/test/plugin/test_output_as_buffered.rb
@@ -75,6 +75,31 @@ def try_write(chunk)
     end
   end
   class DummyCustomFormatBufferedOutput < DummyBareOutput
+    def initialize
+      super
+      @format_type_is_msgpack = nil
+      @prefer_delayed_commit = nil
+      @write = nil
+      @try_write = nil
+    end
+    def format(tag, time, record)
+      @format ? @format.call(tag, time, record) : [tag, time, record].to_json
+    end
+    def formatted_to_msgpack_binary?
+      @format_type_is_msgpack ? @format_type_is_msgpack.call : false
+    end
+    def prefer_delayed_commit
+      @prefer_delayed_commit ? @prefer_delayed_commit.call : false
+    end
+    def write(chunk)
+      @write ? @write.call(chunk) : nil
+    end
+    def try_write(chunk)
+      @try_write ? @try_write.call(chunk) : nil
+    end
+  end
+  # check for formatted_to_msgpack_binary compatibility
+  class DummyOldCustomFormatBufferedOutput < DummyBareOutput
     def initialize
       super
       @format_type_is_msgpack = nil
@@ -163,6 +188,7 @@ def create_output(type=:full)
     when :full     then FluentPluginOutputAsBufferedTest::DummyFullFeatureOutput.new
     when :old_buf  then FluentPluginOutputAsBufferedTest::DummyOldBufferedOutput.new
     when :old_obj  then FluentPluginOutputAsBufferedTest::DummyOldObjectBufferedOutput.new
+    when :old_custom then FluentPluginOutputAsBufferedTest::DummyOldCustomFormatBufferedOutput.new
     else
       raise ArgumentError, "unknown type: #{type}"
     end
@@ -306,9 +332,11 @@ def waiting(seconds)
       assert_equal 0, events_from_chunk.size
     end
 
-    test 'plugin using custom format can iterate chunk in #write if #format returns msgpack' do
+    data('formatted_to_msgpack_binary?' => :custom,
+         'formatted_to_msgpack_binary' => :old_custom)
+    test 'plugin using custom format can iterate chunk in #write if #format returns msgpack' do |out_type|
       events_from_chunk = []
-      @i = create_output(:custom)
+      @i = create_output(out_type)
       @i.configure(config_element('ROOT','',{},[config_element('buffer','',@hash)]))
       @i.register(:prefer_delayed_commit){ false }
       @i.register(:format){ |tag, time, record| [tag,time,record].to_msgpack }