A power supply device comprises plural battery cells having electrode portions, and bus bars connecting the electrode portions of the plural battery cells each other. The bus bar comprises a thin portion thinner than the other portion formed in at least one part of the end edge of the bus bar, and are welded to the electrode portion of the battery cell through the thin portion. The electrode portion comprises a pedestal portion, and an electrode terminal projecting from the pedestal portion, and the thin portion is disposed at the side surface of the electrode terminal. By this, at the time of welding the bus bar the thin portion is directly welded to the electrode portion, and without using other parts of the welding ring or the like the welding process is streamlined.