The present invention developed a novel antitumor agent for renal cell carcinoma that cannot be fully treated by the existing therapies. The present invention provides an antitumor agent comprising a combination drug of tegafur/gimeracil/oteracil potassium that ensures an excellent therapeutic effect in renal cell carcinoma patients that is superior to that of the existing therapies, by way of selecting the patients based on thymidylate synthase.
