Provided is a novel combination therapy using an FTD/TPI combination drug which exhibits remarkable antitumor effects, and few side effects. This antitumor agent is characterized in that one cycle of an administration schedule, in which, in a period of 14 days, the FTD/TPI combination drug is administered on days 1 to 5, and CPT-11 is administered on day 1, is repeated at least once.