The present invention provides an antitumor effect potentiator for enhancing antitumor activity of an antitumor agent comprising tegafur in a therapeutically effective amount, gimeracil in an amount effective for enhancing an antitumor effect, and oteracil potassium in an amount effective for inhibiting a side effect. The antitumor effect potentiator comprising at least one member selected from the group consisting of folinic acid and pharmacologically acceptable salts thereof. The present invention also provides a method for enhancing an antitumor effect of an antitumor agent comprising the step of administering to a patient the aforementioned antitumor effect potentiator; and a method for cancer treatment comprising the step of administering to a patient the aforementioned antitumor agent.