The purpose of the present invention is to provide an antitumor agent for biliary cancer which exhibits an effect against biliary cancer, and a treatment method of biliary cancer. An antitumor agent for biliary cancer is provided that contains 1-(2-deoxy-2-fluoro-4-thio-[beta]-D-arabinofuranosyl)cytosine or a salt or a prodrug thereof.本發明的課題為提供一種對膽道癌示出效果的膽道癌用抗腫瘤劑、和膽道癌的處理方法。根據本發明，提供一種包含1-(2-脫氧-2-氟-4-硫代-β-D-阿拉伯呋喃糖基)胞嘧啶或其鹽或者前驅藥的膽道癌用抗腫瘤劑。
