js拖动div 当鼠标移动时整个div也相应的移动

HTML代码如下，其中，要拖动的div为最外层的div

<div id="dialog_createUserGroup" class="dialog_main" style="">
<div id="McreateUserGroup">
<div class="title">
<span class="poptitle">新建用户组</span>
<span class="dialog_close" title="关闭" onClick="closeUserGroup();">
<img src='/images/close.png' alt='close' class='user_img' title='关闭'/>
</span>
</div>
<!--新建联系人-->
<div class="popContent">
<span class="localinfo" style="padding:10px 10px 0 46px;">组名称</span>
<input type="text" id="userGroupName" name="userGroupName" class="input_info" value="" style="width:265px;">
<img src='/images/ok.png' alt='ok' class='user_img' title='ok' style="display: none;" id="email_ok"/>
<br>
<span id="userGroupName_info" style="margin-left: 100px; color: red;">&nbsp;</span>
<br/>
<span class="localinfo" style="padding:10px 10px 0 46px;margin-top:20px">用&nbsp;&nbsp;&nbsp;户</span>
<div style="padding-top: 20px;width:265px;display:inline-block">
<input type="text" id="group_username" name="group_username" class="input_info" value="" style="width:280px;">
</div>
<br>
<span id="name_info" style="margin-left: 100px; color: red;">&nbsp;</span>
<br>
<span class="localinfo" style="padding:10px 10px 0 46px;">描&nbsp;&nbsp;&nbsp;述</span><br>
<textarea id="userGroup_displayname" class="textarea_comm" rows="5" name="userGroup_displayname" style="width:265px;margin-left: 100px;"></textarea>
<br>
<br>
<br>
<br>
<a href="javascript:void(0);" id="save_contact_btn" class="dialog_btn2" onclick="saveUserGroup();" style="margin-right: 260px;">确定</a>
<a href="javascript:void(0);" class="dialog_btn2" onclick="closeUserGroup();">取消</a>
</div>

js代码如下,

$("#McreateUserGroup").mousedown(function(e){
var flag = true;
e = e||event;
var $dialog_createUserGroup = $("#dialog_createUserGroup");
var LEFT = e.clientX - parseInt($dialog_createUserGroup.css("left")),
TOP = e.clientY - parseInt($dialog_createUserGroup.css("top"));
$(document).mousemove(function (e) {
e = e || event;
if (flag) {
$dialog_createUserGroup.css({
"left": e.clientX - LEFT + "px",
"top": e.clientY - TOP + "px"
});
}
});
$(document).mouseup(function (e) {
flag = false;
});
});

这段代码对显示对话框的头部绑定鼠标监听事件，当鼠标移动时，整个div也相应的移动！