To reduce the tolerance of the cleaving pressure by punching a part to be cleaved with pressing, returning the punched part to the original position, and forming the boundary part with the airtight operation.

After positioning a peripheral part of a battery in a die set, an upper punch is lowered so as to punch a part to be cleaved. Thereafter, a lower punch is raised so as to return the part to be cleaved to the original position. A part of the peripheral part of the battery is punched into a disk shape, and returned to the original position, and a boundary part 6 of the punched part is welded into a ring shape so as to secure air-triteness. Cleaving pressure can be adjusted by adjusting the welding depth of the welding. In the case where the broken-out face of the boundary part 6 is welded remarkably shallow, the cleaving pressure is low, and in the case where the broken-out section is welded deep, the cleaving pressure is high. As a method of airtight operation, besides welding, resin covering method, compound method, resin impregnating method, and plating can be included.