A power source device comprises: a battery module having a rectangular parallelepiped shape including a battery stacked body having a plurality of battery cells stacked in one direction, a pair of end plates respectively disposed on a first end surface and a second end surface located at two ends of the battery stacked body, and a constraining member coupled to the pair of the end plates. Further, the power source device comprises: a frame having a fastening surface; fastening members for fastening the battery module in such a state that one surface of the battery module adjacent to the first end face and the second end face faces the fastening surface. The constraining member is formed such that a hardness of the constraining member becomes stronger against an external force applied to a stacked direction of the plurality of battery cells as the constraining member goes away from the fastening surface.