A power source device includes: a battery stack including a plurality of stacked battery cells each provided with positive and negative electrode terminals; and a bus bar connected to the electrode terminals of the plurality of battery cells to connect the plurality of battery cells in parallel and in series. The bus bar includes a series connection line connecting in series parallel battery groups each including the plurality of battery cells connected in parallel, and a branched connection part branched and connected to each of both ends of the series connection line, in which the electrode terminals of the plurality of battery cells configuring each of the parallel battery groups are connected to the branched connection part to allow the battery cells configuring the parallel battery group to be connected in parallel via the branched connection part.