function changeCity() {
    var pref_id = $('pref').value;
    var city_id = 0;

    $A(document.getElementsByTagName("script")).findAll( function(s) {
        var r = s.src.match(/change_city\.js\?city_id=(.*)?$/);
        if (r != null) {
            city_id = r[1];
            return false;
        }
    });
    ajax = new Ajax.Updater(
            'city',
            'index.php',
            {
                method: 'get',
                parameters: 'action_AjaxCity=1&pref_id=' + pref_id + '&city_id=' + city_id
            });
}

Event.observe(window, "load", function() {
    changeCity();
    Event.observe("pref", "change", changeCity);
});