The present invention provides an antitumor effect potentiator, a method for treating cancer using a plurality of pharmaceutical preparations having excellent antitumor activity, and an antitumor preparation. In particular, the present invention provides an antitumor effect potentiator for potentiating the antitumor activity of an antitumor preparation comprising tegafur in a therapeutically effective amount, gimeracil in an amount effective for potentiating an antitumor effect, and oteracil potassium in an amount effective for inhibiting a side effect, the antitumor effect potentiator comprising cis-oxalate( 1R , 2R -diaminocyclohexane)platinum(II) in an amount effective for potentiating the antitumor effect； a method for treating cancer comprising the step of concomitantly administering tegafur in a therapeutically effective amount, gimeracil in an amount effective for potentiating an antitumor effect, oteracil potassium in an amount effective for inhibiting a side effect, and cis-oxalate( 1R,2R- diaminocyclohexane)platinum(II) in an amount effective for potentiating the antitumor effect； an antitumor preparation in a pharmaceutical form comprising a plurality of pharmaceutical agents each of which contains one of the active ingredients consisting of tegafur, gimeracil, oteracil potassium, and cis-oxalate( 1R , 2R -diaminocyclohexane)platinum(II), or each of which contains such active ingredients in any combination, or in a pharmaceutical form comprising a single pharmaceutical agent containing all the active ingredients； and a kit.
