The present invention provides a method for predicting the therapeutic effect of chemotherapy using an antitumor agent containing trifluridine and tipiracil hydrochloride at a molar ratio of 1:0.5 on a colorectal cancer patient, the method comprising the following steps (1) to (3). As the means for solving the problem, the following invention is provided: a method comprising (1) detecting the expression of TK1 protein by an immunohistochemical method in tumor cells contained in a biological sample obtained from the patient (2) based on the detection results obtained in step (1), classifying the tumor cells into positive cells and negative cells, and calculating the percentage of positive cells in the tumor cells and (3) based on the calculation results obtained in step (2), predicting that when the percentage is 30% or more, the patient is likely to sufficiently respond to chemotherapy using the antitumor agent containing trifluridine and tipiracil hydrochloride at a molar ratio of 1:0.5.
