$(function(){
        var fotter_ul = $("nav#footerContent ul");
        var fade_in_flg = 0;

        var page_top_li = document.createElement('li');
        page_top_li.id = "page_top_li";

        var page_top_a = document.createElement('a');
        page_top_a.id = "page_top_a";
        page_top_a.innerHTML = "ページ先頭へ↑";


        $(window).scroll(function () {  
          if($(window).scrollTop() > 550)
          {
            if(fade_in_flg == 0)
            {
              $(fotter_ul).append(page_top_li);
              $(page_top_li).append(page_top_a);
            }
            fade_in_flg = 1;
          }
            else
          {
            fade_in_flg = 0;
            $(page_top_li).remove();
          }
        });

        function scroll_screen(target_height)
        {
          $('html,body').animate({ scrollTop: target_height }, 'normal');
        }
        $("nav#footerContent").on('click',"#page_top_a", function() {
          scroll_screen(0);
          return false;
        });

});