An object of the present invention is to provide an antitumor agent for bladder cancer and a method for treating bladder cancer, which have an effect on bladder cancer. According to the present invention, there is provided an antitumor agent for bladder cancer, which comprises 1- (2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl) cytosine or a prodrug thereof.本發明的課題在於提供一種對膀胱癌顯現效果之膀胱癌用抗腫瘤劑及膀胱癌的處置方法。依據本發明，提供一種膀胱癌用抗腫瘤劑，其包含1-（2-去氧-2-氟-4-硫-β-D-阿拉伯呋喃糖基）胞嘧啶或其鹽或者前驅藥。
