When a short-circuit failure of any of switch portions (13) including switch elements (11) and parallel-connected feedback diodes (12) of an inverter circuit (7) is detected during the operation of a motor (1), a switch portion (13) where the short-circuit failure has occurred is checked for whether it is on the positive polarity side or the negative polarity side. The switch elements (11) are so controlled that all the switch portions (13) on the same polarity side as where the short-circuit has occurred are brought into a conducted state and all the others are disconnected. This prevents a large electric current from flowing into each switch portion of the inverter circuit without requiring any switch to block the power distribution between a motor and the inverter circuit when a short-circuit failure of the switch portion of the inverter circuit occurs.