A first terminal of a protection element of a control unit is connected to a wire between a motor and a switching element, and a second terminal is directly connected to a ground-side wire shared by a controller and the switching element. The protection element has a normal-condition-OFF-type switch including a movable contact and a fixed contact. When an overheating caused by the switching element attains a predetermined temperature or higher, the movable contact and the fixed contact are closed to short-circuit the first terminal and the second terminal, and an electric current is branched to the protection element side to reduce an electric current flowing into the switching element, and the control unit is shifted to a temperature range safe from the overheating caused by the switching element without stopping a cooling system.