Power supply device includes: a plurality of secondary battery cells each constituted by closing an open end of quadrangular exterior can opened at one end, with sealing plate; insulating separator interposed between adjacent secondary battery cells; one pair of end plates covering both end faces of battery stack constituted by stacking the plurality of secondary battery cells; and a plurality of fastening members fastening end plates together. Power supply device is configured such that within a region where separator is in contact with secondary battery cell, separator is made less easily deformable in a region on the side of sealing plate than other regions. As a result, a portion of separator in contact with the side of sealing plate is made not to easily deform, and therefore, when secondary battery cell expands, a joint portion between sealing plate and exterior can is protected, and meanwhile, exterior can is allowed to deform at other portions, and thereby, the deformation during expansion can be mitigated.