A metal foil extending outward from electrode plates stacked in a layered manner is sandwiched between a first metal member provided with damping portions and a second metal member, and ultrasonic vibration is caused to act upon a plurality of sites of action from the first metal member side to join the metal foil to the first and second metal members. Each damping portion includes at least one of a cutaway portion formed by cutting away part of the first metal member, a thick-walled portion formed by partially increasing the plate thickness of the first metal member, and a convex ridge portion formed by bending part of the first metal member into a convex ridge shape.