To provide a method for treating cancer using a pyrazolo[3, 4-d]pyrimidine compound or a salt thereof. The present invention provides an antitumor agent comprising a pyrazolo[3, 4-d]pyrimidine compound of formula (I) wherein X, Y, Z1, Z2, Z3, Z4, W, n, R1, R2, and R3 have meanings as defined in the present specification, or a salt thereof and other antitumor agent(s) for combined administration.