Provided is a novel combination therapy using an FTD/TPI combination drug which exhibits remarkable antitumor effects, and few side effects.An antitumor agent is characterized in that the FTD/TPI combination drug is administered, using a reduced amount of trifluridine, at a dose in the range of 35 to 70 mg/m2/day and CPT-11 is administered at a dose in the range of 45 to 144 mg/m2/day.