function load_ranking( url ){
  if( ! url ){
     url = "/ranking/ajax";
  }
  var prefecture_id = document.getElementById("ranking_prefecture_id").value;
  new Ajax.Request( url , {
    method:  'get',
    parameters: "prefecture_id=" + prefecture_id ,
    onSuccess:  function(transport) {
      var ranking_table = document.getElementById("ranking_table");
	  if( ranking_table != null ){
		ranking_table.innerHTML = transport.responseText;
	  }
    }
  });
    
}