A battery pack includes a first frame, a second frame, and assembled batteries. The first frame includes a top edge and a bottom edge. The top edge includes an inner projecting piece, an outer projecting piece distanced from the inner projecting piece, an outer depressed part, and an inner depressed part. The bottom edge includes a center projecting piece that is disposed in a position corresponding to a gap between the inner projecting piece and the outer projecting piece when the bottom edge is engaged with the top edge of another first frame and that has a thickness fitting this gap, and an outer engagement projection and an inner engagement projection provided in positions corresponding to the outer depressed part and the inner depressed part.