A method for predicting therapeutic efficacy of a chemotherapy using an antitumor agent comprising a PI3K/AKT/mTOR inhibitor on a cancer patient on the basis of an expression level of PIK3C2B in tumor cells isolated from the cancer patient, comprising the following steps and: measuring an expression level of PIK3C2B in a biological sample containing tumor cells isolated from the patient; and predicting that the patient is likely to sufficiently respond to the chemotherapy using the antitumor agent comprising a PI3K/AKT/mTOR inhibitor when the expression level of PIK3C2B measured in step is equal to or higher than a predetermined cutoff point; wherein the PI3K/AKT/mTOR inhibitor is an imidazo-oxazine compound represented by formula or its pharmaceutically acceptable salt wherein A, B, C, and D represent C-R 1a, C-R 1b, C-R 1c, and C-R 1d, respectively, or one or two of the thus-defined A, B, C, and D are replaced by nitrogen; at least two of R 1a, R 1b, R 1c, and R 1d represent hydrogen, and the other(s) each represent(s)halogen, cyano, C 1-6 alkyl optionally substituted with one or more hydroxyl groups, C 1-6 alkoxy, carbonyl having hydroxyl, amino, optionally substituted mono- or di(C 1-6 alkyl) amino or mono- or di(C 1-6 alkoxy)amino as a substituent, or unsaturated heterocyclic group; R 2 represents phenyl, pyridyl, or thienyl; R 3 represents hydrogen, methyl, ethyl, or cyclopropyl; and R 4 represents hydrogen or hydroxyl; wherein the predetermined cutoff point for the expression level of PIK3C2B is the mean or median value of the PIK3C2B expression levels of cancer patients, or a value at which the p-value of the log-rank test conducted for the therapeutic efficacy is less than 0.1. measuring an expression level of PIK3C2B in a biological sample containing tumor cells isolated from the patient; and predicting that the patient is likely to sufficiently respond to the chemotherapy using the antitumor agent comprising a PI3K/AKT/mTOR inhibitor when the expression level of PIK3C2B measured in step is equal to or higher than a predetermined cutoff point; wherein the PI3K/AKT/mTOR inhibitor is an imidazo-oxazine compound represented by formula or its pharmaceutically acceptable salt wherein A, B, C, and D represent C-R 1a, C-R 1b, C-R 1c, and C-R 1d, respectively, or one or two of the thus-defined A, B, C, and D are replaced by nitrogen; at least two of R 1a, R 1b, R 1c, and R 1d represent hydrogen, and the other(s) each represent(s)halogen, cyano, C 1-6 alkyl optionally substituted with one or more hydroxyl groups, C 1-6 alkoxy, carbonyl having hydroxyl, amino, optionally substituted mono- or di(C 1-6 alkyl) amino or mono- or di(C 1-6 alkoxy)amino as a substituent, or unsaturated heterocyclic group; R 2 represents phenyl, pyridyl, or thienyl; R 3 represents hydrogen, methyl, ethyl, or cyclopropyl; and R 4 represents hydrogen or hydroxyl; wherein the predetermined cutoff point for the expression level of PIK3C2B is the mean or median value of the PIK3C2B expression levels of cancer patients, or a value at which the p-value of the log-rank test conducted for the therapeutic efficacy is less than 0.1. A, B, C, and D represent C-R 1a, C-R 1b, C-R 1c, and C-R 1d, respectively, or one or two of the thus-defined A, B, C, and D are replaced by nitrogen; at least two of R 1a, R 1b, R 1c, and R 1d represent hydrogen, and the other(s) each represent(s)halogen, cyano, C 1-6 alkyl optionally substituted with one or more hydroxyl groups, C 1-6 alkoxy, carbonyl having hydroxyl, amino, optionally substituted mono- or di(C 1-6 alkyl) amino or mono- or di(C 1-6 alkoxy)amino as a substituent, or unsaturated heterocyclic group; R 2 represents phenyl, pyridyl, or thienyl; R 3 represents hydrogen, methyl, ethyl, or cyclopropyl; and R 4 represents hydrogen or hydroxyl; wherein the predetermined cutoff point for the expression level of PIK3C2B is the mean or median value of the PIK3C2B expression levels of cancer patients, or a value at which the p-value of the log-rank test conducted for the therapeutic efficacy is less than 0.1.