A control unit determines a presence or absence of a short-circuit failure with respect to a target switch which is a phase-opening switch to be determined. At this time, after opening all the phase opening switches, the control unit drives an inverter to connect a power supply line provided with the target switch among the phase opening switches to ground. The control unit determines that the target switch has a short-circuit failure when a voltage of the power supply line in which the target switch is not provided is smaller than a predetermined voltage.