How to Disable LookUp Input Field on Visualforce Page?

​Sample Code :

<apex:page standardController="Contact" id="page">

    <apex:form id="form">

        <apex:inputField value="{!Contact.AccountId}" id="AccId" />

    </apex:form>

    <script>

    var inp = document.getElementById("page:form:AccId");

    inp.setAttribute("readonly","true");   

    </script>  

</apex:page>