Abstract: An object of the present invention is to provide a chemotherapy using a PI3K/AKT/mTOR inhibitor that produces an excellent antitumor effect on a cancer patient. The present invention provides a method for predicting the therapeutic efficacy of the chemotherapy using the antitumor agent comprising a PI3K/AKT/mTOR inhibitor on a cancer patient on the basis of the expression level of PHLDA1 and/or PIK3C2B in a biological sample containing tumor cells isolated from the cancer patient.