A superior antitumor agent is provided. A salt of 1-(2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosine shows at least one or more of such characteristics as (1) it has superior antitumor activity, (2) it shows superior crystallinity, (3) it shows high water solubility, (4) it does not show deliquescent property, (5) it shows superior flowability, (6) it shows superior tableting property, (7) it can be manufactured with less environmental load, and (8) it can be manufactured in a large scale, and therefore it is useful as a bulk drug for medicaments.本發明係以提供一種優異之抗腫瘤劑作為課題。1-(2-脫氧-2-氟-4-硫-β-D-阿拉伯呋喃糖基)胞嘧啶之鹽，係因具有：(1)具有優異之抗腫瘤活性、(2)結晶性優異、(3)對於水之溶解度高、(4)不顯示潮解性、(5)流動性優異、(6)打錠性優異、(7)以環境負荷少之條件製造、或(8)大量製造等之至少1個以上之特徵，而有用於作為醫藥品之前藥。
