diff --git a/lib/sender.js b/lib/sender.js
index d0a3a60..93380ed 100644
--- a/lib/sender.js
+++ b/lib/sender.js
@@ -26,10 +26,13 @@ function FluentSender(tag_prefix, options){
   this._timeResolution = options.milliseconds ? 1 : 1000;
   this._socket = null;
   this._data = null;
-  this._sendQueue  = [];    // queue for items waiting for being sent.
+  this._sendQueue = []; // queue for items waiting for being sent.
+  this._outstandingChunks = {}; // chunks we haven't gotten a response for, indexed by chunk id
   this._sendQueueTail = -1;
   this._eventEmitter = new EventEmitter();
+  this._handleAck = this._handleAck.bind(this)
   this._flushSendQueue = this._flushSendQueue.bind(this)
+  this._setupAckTimeout = this._setupAckTimeout.bind(this)
 }
 
 FluentSender.prototype.emit = function(/*[label] <data>, [timestamp], [callback] */){
@@ -157,9 +160,7 @@ FluentSender.prototype._connect = function(callback){
     self._socket.on('connect', function() {
       self._handleEvent('connect');
     });
-    self._socket.on('data', function(data) {
-      self._data = data;
-    });
+    self._socket.on('data', self._handleAck);
     if (self.path) {
       self._socket.connect(self.path, callback);
     } else {
@@ -178,6 +179,25 @@ FluentSender.prototype._connect = function(callback){
   }
 };
 
+FluentSender.prototype._handleAck = function(data) {
+  var item, response = msgpack.decode(data, {codec: codec});
+  if ((item = this._outstandingChunks[response.ack]) == null) {
+    throw new Error('unmatched chunk id ' + response.ack)
+  } else {
+    clearTimeout(item.timeoutId);
+    delete this._outstandingChunks[response.ack];
+    item.callback && item.callback();
+  }
+}
+
+FluentSender.prototype._setupAckTimeout = function(item) {
+  var self = this;
+  item.timeoutId = setTimeout(function() {
+    var error = new FluentLoggerError.ResponseTimeout('ack response timeout');
+    self._handleEvent('error', error, item.callback);
+  }, this.ackResponseTimeout);
+}
+
 FluentSender.prototype._flushSendQueue = function() {
   var self = this;
   var pos = self._sendQueue.length - self._sendQueueTail - 1;
@@ -187,28 +207,12 @@ FluentSender.prototype._flushSendQueue = function() {
   } else {
     self._sendQueueTail--;
     self._sendQueue.shift();
-    self._socket.write(new Buffer(item.packet), function(){
-      if (self.requireAckResponse) {
-        var intervalId = setInterval(function() {
-          if (self._data) {
-            var response = msgpack.decode(self._data, { codec: codec });
-            self._data = null;
-            clearInterval(intervalId);
-            clearTimeout(timeoutId);
-            if (response.ack !== item.options.chunk) {
-              var error = new FluentLoggerError.ResponseError('ack in response and chunk id in sent data are different',
-                                                              { ack: response.ack, chunk: item.options.chunk });
-              self._handleEvent('error', error, item.callback);
-            }
-          }
-        }, 100);
-        var timeoutId = setTimeout(function() {
-          var error = new FluentLoggerError.ResponseTimeout('ack response timeout');
-          self._handleEvent('error', error, item.callback);
-          clearInterval(intervalId);
-        }, self.ackResponseTimeout);
-      }
-      item.callback && item.callback();
+    if (self.requireAckResponse) {
+      self._outstandingChunks[item.options.chunk] = item;
+      self._setupAckTimeout(item);
+    }
+    self._socket.write(new Buffer(item.packet), function() {
+      if (!self.requireAckResponse) item.callback && item.callback();
     });
     process.nextTick(function() {
       // socket is still available