This invention provides a method for predicting a therapeutic effect of chemotherapy that uses an antitumor agent comprising α,α,α-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).
