A control device determines whether a motor generator is controlled in a PWM control mode, an overmodulation control mode or a rectangular-wave control mode. If a command to perform a boosting operation by a voltage step-up converter is issued while the motor generator is controlled in the rectangular-wave control mode, the control device controls an inverter to drive the motor generator by switching the control mode to the overmodulation or PWM control mode. Further, the control device controls the inverter to drive the motor generator by suppressing increase of a torque command value.