Disclosed are an antitumor agent comprising (1) a combination preparation of tegafur, gimeracil, and oteracil potassium, (2) at least one component selected from the group consisting of folinic acid and pharmaceutically acceptable salts thereof, and (3) cis-oxalate(1R,2R-diaminocyclohexane)platinum(II) in combination； a kit comprising a combination of a pharmaceutical composition for treating cancer in a mammal comprising (a) a combination preparation of tegafur, gimeracil, and oteracil potassium in an amount effective for treatment, (b) a composition containing at least one component selected from the group consisting of folinic acid and pharmaceutically acceptable salts thereof in an amount effective for potentiating an antitumor effect, and (c) a composition containing cis-oxalate(1R,2R-diaminocyclohexane)platinum(II) in an amount effective for potentiating an antitumor effect； and a method for treating cancer.
