function window_center(str_url, num_window_width, num_window_height) {

    var num_screen_width, num_screen_height, obj_window;

    num_screen_width  = screen.width;
    num_screen_height = screen.height;

    num_position_x = (num_screen_width - num_window_width) / 2;
    num_position_y = (num_screen_height - num_window_height) / 2;

    obj_window = window.open(str_url, 'window_center', 'left=' + num_position_x + ',top=' + num_position_y + ',screenX=' + num_position_x + ',screenY=' + num_position_y + ',width=' + num_window_width + ',height=' + num_window_height + ',menubar=no,toolbar=no');
    obj_window.focus();
}