undefined $ error in jQuery function

I want to write my jquery function

function insert_post()
{
		$.post('ajax_post_insert.php' , {}
		,function(str)
		{
			alert(str);
			$("#load_gallery").html(str);}
		);
}

But its giving JS error ‘$ is undefined’.The jquery.js file is also included,i confirmed it.How can i achieve this functionality in wordpress ?
Thanks.