A power supply device having a battery block with a terminal surface on which terminals of a plurality of battery cells are arranged side by side, and a side plate with an elastic part, a holding part and a body part that extends along the side surfaces of the battery block. The elastic part faces the terminal surface and the holding part faces the bottom surface of the battery block. An insulation part is disposed between the elastic part and the battery block and has a positioning region and a pressing region that are disposed on one surface of the insulation part so as to face the elastic part. The positioning region is provided at an end of the insulation part located at the body part side, and extends to a position that is separated farther away from the elastic part than the pressing region.