An object of the present invention is to further enhance the antitumor effect when oxaliplatin is administered in combination with a combination drug containing tegafur, gimeracil and oteracil potassium. According to the present invention, by using oxaliplatin encapsulated in a liposome preparation, combination therapy of oxaliplatin plus a combination drug containing tegafur, gimeracil and oteracil potassium, is revealed to show remarkably enhanced antitumor effect without increasing side effects.