A transformer includes a tank that is attached under a floor of a vehicle, the tank being for housing an iron core, a coil, and an insulating liquid, thereby immersing the iron core and the coil in the insulating liquid; and a cooling unit for air-cooling the insulating liquid that has flowed from tank and returning the insulating liquid to tank, the cooling unit including a plurality of pipes arranged to be spaced from one another such that a spacing in a region relatively closer to floor of the vehicle is greater than a spacing in a region relatively away from floor of the vehicle.