A water pump unit 7 has, within a water pump case 72, a drive shaft 71 for driving an impeller, and a transmission member 74 for transmitting power of a power supply device 3 to the drive shaft 71. The water pump unit 7 is configured such that the water pump case 72 receives therein and supports the drive shaft 71, the transmission member 74 is supported by the water pump case 72 by means of the drive shaft 71 with a part of the transmission member 74 protruding from within the water pump case 72, and an attachment surface 72c of the water pump case 72, for joining the water pump case to the power supply device 3, is formed to extend along the transmission member 74.