The power supply device includes: a plurality of secondary battery cells each including a prismatic exterior can; a pair of end plates covering both end surfaces of a battery stack body in which the plurality of secondary battery cells are stacked; and a plurality of fastening members made of metal each having a plate shape extending in a stack direction of the plurality of secondary battery cells and disposed on an opposing side surface of the battery stack body to fasten the end plates to each other, in which each of the plurality of fastening members includes fastening portion fixed to the end plate at each of both ends in a longer direction, and intermediate portion coupling fastening portions with each other, and fastening portions is higher in strength than intermediate portion, and intermediate portion is higher in stretchability than fastening portions.