The present invention provides a method for predicting the therapeutic effect of chemotherapy using an anti-tumor agent that contains trifluridine and tipiracil hydrochloride at a molar ratio of 1:0.5 on a colorectal cancer patient, said method comprising the steps (1)-(3) that are indicated below. As a means of solving the problem addressed by the present invention, a method comprising the following steps is provided: a step (1) in which the expression of the TK1 protein in the cytoplasm of a tumor cell that is included in a biological sample taken from the patient is detected using immunohistochemistry； a step (2) in which tumor cells are classified as positive cells and negative cells from the detection result obtained at step (1) and the share of positive cells among the tumor cells is calculated； and a step (3) in which, when the share is 30% or higher, it is predicted from the calculation result obtained at step (2) that there is a high possibility that chemotherapy using an anti-tumor agent that contains trifluridine and tipiracil hydrochloride at a molar ratio of 1:0.5 will have a sufficient therapeutic effect on the patient.
