1- (2-deoxy-2-fluoro-4-thio-beta-D-arabinofuranosyl) salt of cytosine

And to provide an excellent antitumor agent. 1- salts (2-deoxy-2-fluoro-4-thio-beta-D-arabinofuranosyl) cytosine has (1) superior anti-tumor activity, excellent (2) crystalline, (3 ) is highly soluble in water, (4) do not exhibit deliquescence, (5) excellent fluidity, (6) excellent in compressibility, (7) or environmental impact is produced with less condition (8) mass is produced, it is because it has at least one or more characteristics, such as, useful as a pharmaceutical drug substance.