A controller of a power transmission system for a vehicle includes an electronic control unit. When a difference between a secondary pressure set by use of a command pressure of the electromagnetic control valve for the secondary pulley, and an actual pressure obtained by a hydraulic pressure sensor, is larger than a predetermined pressure difference, the electronic control sets a primary pressure and the secondary pressure such that a speed ratio of a continuously variable transmission becomes substantially equal to a maximum value. The electronic control unit determines that there is an abnormality that an output pressure of the electromagnetic control valve for a secondary pulley is low, when the speed ratio is smaller than a predetermined first determination value, and determines that there is an abnormality in the hydraulic pressure sensor, when the speed ratio is larger than a predetermined second determination value.