When a motor rotation speed is equal to or greater than a predetermined rotation speed when an automobile collision is detected or when power stops, a motor rotation speed reduction control is carried out by an inverter control unit, and when an inter-terminal voltage of a secondary smoothing capacitor drops below a predetermined voltage during the motor rotation speed reduction control, the motor rotation speed reduction control is stopped, whereby induction voltage caused by motor rotation is applied to the secondary smoothing capacitor, the inter-terminal voltage of the secondary smoothing capacitor is raised, and the motor rotation speed reduction control is carried out while keeping the inter-terminal voltage of the secondary smoothing capacitor within an operating voltage range of the inverter control unit.