A power supply unit comprises a solid electrolyte battery constituted by using a solid electrolyte alone and a liquid electrolyte battery constituted by using an electrolytic solution connected in parallel to each other. Since the solid electrolyte battery is primarily utilized at high temperature and the liquid electrolyte battery is primarily utilized at low temperature, a power supply unit can achieve durability characteristics at high temperature equivalent to those of a power supply unit constituted of solid batteries alone and output characteristics at low temperatures equivalent to those of liquid electrolyte batteries alone, is provided.