The present invention provides a method for predicting the therapeutic effect of chemotherapy using an antitumor agent that contains α,α,α-trifluorothymidine and 5-chloro-6-(1-(2-iminopyrrolidinyl)methyl) uracil hydrochloride in a molar ratio of 1:0.5 in patients with colorectal cancer, the method comprising steps (1) and (2): (1) a step for detecting the presence or absence of mutation of the KRAS gene contained in a biological sample collected from the patient, and (2) a step for predicting a high likelihood that the chemotherapy will exhibit an adequate therapeutic effect in that patient, when the result confirmed in step (1) shows that the KRAS gene has mutated.
