function tweet() {
    var login  = 'jimonet'; // ← bitly に登録したユーザ名
    var apiKey = 'R_79902f9b1617dbd44ba1e74bff65f9f0'; // ← bitly API Key
    bitly = 'http://api.bit.ly/shorten' 
        + '?version=2.0.1&format=json&callback=callback'
        + '&login=' + login
        + '&apiKey=' + apiKey + '&longUrl=';
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = bitly + encodeURIComponent(location.href) ;
    document.body.appendChild(script);
  }

  function callback(bitlyResponse) {
    var title = document.title;
    var url = 'http://twitter.com/home/?status='
        + encodeURIComponent(
             title + ' '
               + bitlyResponse.results[location.href]['shortUrl']
               + ' '
          );
    location.href = url;
  }