diff --git a/lib/fluent/plugin/.in_monitor_agent.rb.swo b/lib/fluent/plugin/.in_monitor_agent.rb.swo
new file mode 100644
index 0000000000..77d59ce0cb
Binary files /dev/null and b/lib/fluent/plugin/.in_monitor_agent.rb.swo differ
diff --git a/lib/fluent/plugin/in_monitor_agent.rb b/lib/fluent/plugin/in_monitor_agent.rb
index 6d607c913f..9459a66702 100644
--- a/lib/fluent/plugin/in_monitor_agent.rb
+++ b/lib/fluent/plugin/in_monitor_agent.rb
@@ -226,6 +226,11 @@ def initialize
       @first_warn = false
     end
 
+    def configure(conf)
+      super
+      @port += fluentd_worker_id
+    end
+
     def multi_workers_ready?
       true
     end
@@ -233,7 +238,7 @@ def multi_workers_ready?
     def start
       super
 
-      log.debug "listening monitoring http server on http://#{@bind}:#{@port}/api/plugins"
+      log.debug "listening monitoring http server on http://#{@bind}:#{@port}/api/plugins for worker#{fluentd_worker_id}"
       @srv = WEBrick::HTTPServer.new({
           BindAddress: @bind,
           Port: @port,
diff --git a/test/plugin/test_in_monitor_agent.rb b/test/plugin/test_in_monitor_agent.rb
index 0b5a98c6d0..717854b6be 100644
--- a/test/plugin/test_in_monitor_agent.rb
+++ b/test/plugin/test_in_monitor_agent.rb
@@ -204,10 +204,9 @@ def get(uri, header = {})
     unless header.has_key?('Content-Type')
       header['Content-Type'] = 'application/octet-stream'
     end
-    res = Net::HTTP.start(url.host, url.port) {|http|
+    Net::HTTP.start(url.host, url.port) {|http|
       http.request(req)
     }
-    res.body
   end
 
   sub_test_case "servlets" do
@@ -268,7 +267,7 @@ def get(uri, header = {})
       expected_test_filter_response = "\
 plugin_id:test_filter\tplugin_category:filter\ttype:test_filter\toutput_plugin:false\tretry_count:"
 
-      response = get("http://127.0.0.1:#{@port}/api/plugins")
+      response = get("http://127.0.0.1:#{@port}/api/plugins").body
       test_in = response.split("\n")[0]
       test_filter = response.split("\n")[3]
       assert_equal(expected_test_in_response, test_in)
@@ -306,7 +305,7 @@ def get(uri, header = {})
       }
       expected_null_response.merge!("config" => {"@id" => "null", "@type" => "null"}) if with_config
       expected_null_response.merge!("retry" => {}) if with_retry
-      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json"))
+      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json").body)
       test_in_response = response["plugins"][0]
       null_response = response["plugins"][5]
       assert_equal(expected_test_in_response, test_in_response)
@@ -343,7 +342,7 @@ def get(uri, header = {})
       }
       expected_null_response.merge!("config" => {"@id" => "null", "@type" => "null"}) if with_config
       expected_null_response.merge!("retry" => {}) if with_retry
-      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json#{query_param}"))
+      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json#{query_param}").body)
       test_in_response = response["plugins"][0]
       null_response = response["plugins"][5]
       assert_equal(expected_test_in_response, test_in_response)
@@ -376,7 +375,7 @@ def get(uri, header = {})
         "type"            => "null",
         "instance_variables" => {"id" => "null", "num_errors" => 0}
       }
-      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json?with_config=no&with_retry=no&with_ivars=id,num_errors"))
+      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json?with_config=no&with_retry=no&with_ivars=id,num_errors").body)
       test_in_response = response["plugins"][0]
       null_response = response["plugins"][5]
       assert_equal(expected_test_in_response, test_in_response)
@@ -394,7 +393,7 @@ def get(uri, header = {})
       expected_response_regex = /pid:\d+\tppid:\d+\tconfig_path:\/etc\/fluent\/fluent.conf\tpid_file:\tplugin_dirs:\[\"\/etc\/fluent\/plugin\"\]\tlog_path:/
 
       assert_match(expected_response_regex,
-                   get("http://127.0.0.1:#{@port}/api/config"))
+                   get("http://127.0.0.1:#{@port}/api/config").body)
     end
 
     test "/api/config.json" do
@@ -405,7 +404,7 @@ def get(uri, header = {})
   tag monitor
 ")
       d.instance.start
-      res = JSON.parse(get("http://127.0.0.1:#{@port}/api/config.json"))
+      res = JSON.parse(get("http://127.0.0.1:#{@port}/api/config.json").body)
       assert_equal("/etc/fluent/fluent.conf", res["config_path"])
       assert_nil(res["pid_file"])
       assert_equal(["/etc/fluent/plugin"], res["plugin_dirs"])
@@ -474,7 +473,7 @@ def write(chunk)
         output.submit_flush_once
         sleep 0.1 until output.buffer.queued?
       end
-      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json"))
+      response = JSON.parse(get("http://127.0.0.1:#{@port}/api/plugins.json").body)
       test_out_fail_write_response = response["plugins"][1]
       # remove dynamic keys
       response_retry_count = test_out_fail_write_response.delete("retry_count")
@@ -486,4 +485,32 @@ def write(chunk)
       assert{ response_retry_count == response_retry["steps"] + 1 }
     end
   end
+
+  sub_test_case "check the port number of http server" do
+    test "on single worker environment" do
+      port = unused_port
+      d = create_driver("
+  @type monitor_agent
+  bind '127.0.0.1'
+  port #{port}
+")
+      d.instance.start
+      assert_equal("200", get("http://127.0.0.1:#{port}/api/plugins").code)
+    end
+
+    test "worker_id = 2 on multi worker environment" do
+      port = unused_port
+      Fluent::SystemConfig.overwrite_system_config('workers' => 4) do
+        d = Fluent::Test::Driver::Input.new(Fluent::Plugin::MonitorAgentInput)
+        d.instance.instance_eval{ @_fluentd_worker_id = 2 }
+        d.configure("
+  @type monitor_agent
+  bind '127.0.0.1'
+  port #{port - 2}
+")
+        d.instance.start
+      end
+      assert_equal("200", get("http://127.0.0.1:#{port}/api/plugins").code)
+    end
+  end
 end