In a predetermined operating state of an electric motor of an axial air-gap type, control is carried out as follows. A phase difference between an electric current supplied to an armature winding of one of the stators and an electric current supplied to an armature winding of the other stator is set so as to suppress a variation in an output torque of the electric motor, and electric currents having the phase difference are supplied to the armature windings of the stators. A variation in output torque can be suppressed only in an operating state where the variation in output torque of the electric motor is to be suppressed, and the output torque and the energy efficiency of the electric motor can be enhanced sufficiently in the other operating states.