diff --git a/lib/fluent/event.rb b/lib/fluent/event.rb
index db644ccf4d..bcb06998ed 100644
--- a/lib/fluent/event.rb
+++ b/lib/fluent/event.rb
@@ -308,11 +308,15 @@ def ensure_decompressed!
   end
 
   module ChunkMessagePackEventStreamer
-    # chunk.extend(ChunkEventStreamer)
+    # chunk.extend(ChunkMessagePackEventStreamer)
     #  => chunk.each{|time, record| ... }
     def each(unpacker: nil, &block)
+      # Note: If need to use `unpacker`, then implement it,
+      # e.g., `unpacker.feed_each(io.read, &block)` (Not tested)
+      raise NotImplementedError, "'unpacker' argument is not implemented." if unpacker
+
       open do |io|
-        (unpacker || Fluent::MessagePackFactory.msgpack_unpacker(io)).each(&block)
+        Fluent::MessagePackFactory.msgpack_unpacker(io).each(&block)
       end
       nil
     end
diff --git a/test/plugin/test_buffer_chunk.rb b/test/plugin/test_buffer_chunk.rb
index 5a1bfeb9da..535b0acd1a 100644
--- a/test/plugin/test_buffer_chunk.rb
+++ b/test/plugin/test_buffer_chunk.rb
@@ -57,6 +57,17 @@ class BufferChunkTest < Test::Unit::TestCase
       assert chunk.respond_to?(:msgpack_each)
     end
 
+    test 'unpacker arg is not implemented for ChunkMessagePackEventStreamer' do
+      meta = Object.new
+      chunk = Fluent::Plugin::Buffer::Chunk.new(meta)
+      chunk.extend Fluent::ChunkMessagePackEventStreamer
+
+      unpacker = Fluent::MessagePackFactory.thread_local_msgpack_unpacker
+
+      assert_raise(NotImplementedError){ chunk.each(unpacker: unpacker) }
+      assert_raise(NotImplementedError){ chunk.msgpack_each(unpacker: unpacker) }
+    end
+
     test 'some methods raise ArgumentError with an option of `compressed: :gzip` and without extending Compressble`' do
       meta = Object.new
       chunk = Fluent::Plugin::Buffer::Chunk.new(meta)