A case is made by disposing side plate portions on the long side and a side plate portion on one short side on the circumference of a plate portion. Provided on the other short side of the case in two tiers are a side plate portion and a substrate casing portion which has a height lower than the side plate portion. The substrate casing portion has a lower plate portion, side plate portions and an upper plate portion. The lower plate portion is provided with window portions from which the respective output terminals are exposed. The battery assembly is held in the case by fitting the protection circuit substrate which is laid down on a side face, into the substrate casing portion.