The present invention provides an antitumor agent comprising cisplatin and a combination drug of tegafur/gimeracil/oteracil potassium that ensures an excellent life-prolongation effect in advanced gastric cancer patients that is superior to that of the standard therapy in Europe and the U.S. using an agent that contains 5-FU and does not contain a dihydropyrimidine dehydrogenase inhibitor, by way of selecting the patients based on dihydropyrimidine dehydrogenase.