A battery voltage is increased in a converter and is input to an inverter that supplies a motor with a motor drive current. A control unit detects input and output voltages of the converter from outputs of voltage sensors and controls the switching in the converter in accordance with the detected input and output voltages. When one of the sensors fails, the control unit estimates the voltage that would have otherwise been detected by the failed voltage sensor based on the switching state in the converter and the voltage detected by the other voltage sensor.