Provided is a novel method for treating a cancer using an FTD/TPI combination drug, which shows remarkably excellent antitumor effect and small adverse effects.An antitumor agent, in which a combination drug containing trifluridine and tipiracil hydrochloride at a molar ratio of 1:0.5 and an anti-VEGF antibody or anti-EGFR antibody are administered in combination.