A control unit of an inverter for driving a three-phase motor receives a voltage of a connection point from a circuit, in which the connection point interconnecting one end of each of three first resistors is connected to the ground through a second resistor, and the other end of each of the three first resistors is connected to each phase. The control unit diagnoses whether the inverter has incurred a failure based on the voltage of the connection point, the voltage being detected when a first switch provided on a positive bus is turned off, a second switch provided in parallel with the first switch and in series with a third resistor is turned on, a third switch provided in series with a fourth resistor between the power source and the connection point is turned on, and the switching elements are turned on or off according to a predetermined pattern.