This invention provides a method for predicting a therapeutic effect of chemotherapy that uses an antitumor agent comprising ±,±,±-trifluorothymidine and 5-chloro-6-(1-(2-iminopyrrolidinyl)methyl)uracil hydrochloride at a molar ratio of 1:0.5 on a colorectal cancer patient, the method comprising: (1) detecting the presence or absence of KRAS gene mutation in a biological sample obtained from the patient； and (2) predicting that the patient is likely to sufficiently respond to the chemotherapy, when KRAS gene mutation is detected in Step (1).
