A battery unit, a bottomed case, a plate-shaped cooling plate, and first heat transfer material are included. The cooling plate is fastened to a bottom part of the case from the outside of the case, and can cool the battery unit via the bottom part of the case. The first heat transfer material have a plastic property and are held between the bottom part of the case and the cooling plate. The bottom part of the case has heat transfer material housings that house excess first heat transfer material therein when the cooling plate has been fastened to the case.