A power supply device according to an aspect of the present invention includes a plurality of battery cells having a flat right-angled parallelepiped shape and a plurality of separators insulating adjacent ones of the plurality of battery cells. Each of the plurality of separators includes heat insulating sheet arranged between adjacent one of the plurality of battery cells and an adjacent cell adjacent to the one of the plurality of battery cells among the plurality of battery cells, and insulating molded member holding heat insulating sheet. Insulating molded member includes lower wall covering lower surfaces of one of the plurality of battery cells and an adjacent cell adjacent to the one of the plurality of battery cells among the plurality of battery cells, and holder holding insulating sheet with a gap disposed between lower wall and insulating sheet.