diff --git a/test/plugin/out_forward/test_ack_handler.rb b/test/plugin/out_forward/test_ack_handler.rb
new file mode 100644
index 0000000000..61c895f568
--- /dev/null
+++ b/test/plugin/out_forward/test_ack_handler.rb
@@ -0,0 +1,95 @@
+require_relative '../../helper'
+require 'fluent/test/driver/output'
+require 'flexmock/test_unit'
+
+require 'fluent/plugin/out_forward'
+require 'fluent/plugin/out_forward/ack_handler'
+
+class AckHandlerTest < Test::Unit::TestCase
+  data(
+    'chunk_id is matched' => [MessagePack.pack({ 'ack' => Base64.encode64('chunk_id 111') }), Fluent::Plugin::ForwardOutput::AckHandler::Result::SUCCESS],
+    'chunk_id is not matched' => [MessagePack.pack({ 'ack' => 'unmatched' }), Fluent::Plugin::ForwardOutput::AckHandler::Result::CHUNKID_UNMATCHED],
+    'chunk_id is empty' => ['', Fluent::Plugin::ForwardOutput::AckHandler::Result::FAILED],
+  )
+  test 'returns node, sock and result status' do |args|
+    receved, state = args
+    ack_handler = Fluent::Plugin::ForwardOutput::AckHandler.new(10, log: $log, read_length: 100)
+    r, w = IO.pipe
+
+    node = flexmock('node', host: '127.0.0.1', port: '1000') # for log
+    chunk_id = 'chunk_id 111'
+    ack = ack_handler.create_ack(chunk_id, node)
+
+    w.write(chunk_id)
+    mock(r).recv(anything) { |_| receved } # IO does not have recv
+    ack.enqueue(r)
+
+    a1 = a2 = a3 = a4 = nil
+    ack_handler.ack_reader(1) do |cid, n, s, ret|
+      # This block is rescued by ack_handler so it needs to invoke assetion outside of this block
+      a1 = cid; a2 = n; a3 = s; a4 = ret
+    end
+
+    assert_equal chunk_id, a1
+    assert_equal node, a2
+    assert_equal r, a3
+    assert_equal state, a4
+  ensure
+    r.close rescue nil
+    w.close rescue nil
+  end
+
+  test 'returns nil if raise an error' do
+    ack_handler = Fluent::Plugin::ForwardOutput::AckHandler.new(10, log: $log, read_length: 100)
+    r, w = IO.pipe
+
+    node = flexmock('node', host: '127.0.0.1', port: '1000') # for log
+    chunk_id = 'chunk_id 111'
+    ack = ack_handler.create_ack(chunk_id, node)
+
+    w.write(chunk_id)
+    mock(r).recv(anything) { |_| raise 'unexpected error' } # IO does not have recv
+    ack.enqueue(r)
+
+    a1 = a2 = a3 = a4 = nil
+    ack_handler.ack_reader(1) do |cid, n, s, ret|
+      # This block is rescued by ack_handler so it needs to invoke assetion outside of this block
+      a1 = cid; a2 = n; a3 = s; a4 = ret
+    end
+
+    assert_equal nil, a1
+    assert_equal nil, a2
+    assert_equal nil, a3
+    assert_equal Fluent::Plugin::ForwardOutput::AckHandler::Result::FAILED, a4
+  ensure
+    r.close rescue nil
+    w.close rescue nil
+  end
+
+  test 'when ack is expired' do
+    ack_handler = Fluent::Plugin::ForwardOutput::AckHandler.new(0, log: $log, read_length: 100)
+    r, w = IO.pipe
+
+    node = flexmock('node', host: '127.0.0.1', port: '1000') # for log
+    chunk_id = 'chunk_id 111'
+    ack = ack_handler.create_ack(chunk_id, node)
+
+    w.write(chunk_id)
+    mock(r).recv(anything).never
+    ack.enqueue(r)
+
+    a1 = a2 = a3 = a4 = nil
+    ack_handler.ack_reader(1) do |cid, n, s, ret|
+      # This block is rescued by ack_handler so it needs to invoke assetion outside of this block
+      a1 = cid; a2 = n; a3 = s; a4 = ret
+    end
+
+    assert_equal chunk_id, a1
+    assert_equal node, a2
+    assert_equal r, a3
+    assert_equal Fluent::Plugin::ForwardOutput::AckHandler::Result::FAILED, a4
+  ensure
+    r.close rescue nil
+    w.close rescue nil
+  end
+end