Power supply device includes a plurality of secondary battery cells, a pair of end plates that cover both end faces of battery stack in which the plurality of secondary battery cells are stacked, a plurality of fastening members that fasten end plates to each other, a plurality of pressing parts that press upper surfaces of the plurality of secondary battery cells respectively, and buffer interposed between pressing parts and the upper surfaces of secondary battery cells. Buffer includes a plurality of covers made of resin, and a connecting part, having flexibility, for connecting two of the plurality of covers. Buffer is configured such that each of the plurality of covers is disposed on the upper surface of corresponding one of secondary battery cells, and each of pressing part abuts on the upper surface of the corresponding cover.