Abstract: The invention provides;an antitumor potentiator for potentiating the antitumor activity of an antitumor composition containing tegafur in a therapeutically effective amount and uracil in an amount effective for potentiation of antitumor effect, characterized by containing a folinic acid or a pharmaceutically acceptable salt thereof in an amount effective for potentiation of antitumor effect as an active ingredient;an antitumor composition characterized by containing tegafur in a therapeutically effective amount, uracil in an amount effective for potentiation of antitumor effect and folinic acid or a pharmaceutically acceptable salt thereof in an amount effective for potentiation of antitumor effect;a method of potentiating the antitumor effect of an antitumor composition characterized by administering the above antitumor potentiator to a patient; anda method for therapy of cancer in mammalian animals characterized by administering the above antitumor composition to a patient.