In a hybrid vehicle or the like, it is necessary to provide a cooling circuit for cooling an electric device such as a battery by cooling water. For example, JP-A-1111-117744 discloses a hybrid electric vehicle which includes a cooling circuit for cooling an electric generator, an inverter, a motor for running and auxiliary devices such as an air compressor and an air conditioning unit by cooling water circulated with a water pump, wherein a sub-tank for storing reserve cooling water is connected to the cooling circuit, and an air piping for introducing compressed air from a compressed air source is connected to an air-venting hole of the sub-tank via a solenoid valve.