A plating layer 4 is formed on a surface of a battery cover 3, and a peripheral edge part 37b of a cover case 37 is arranged on an upper surface of the plating layer 4. A welding part 40 is formed at a tip part of the peripheral edge part 37b. The welding part 40 includes a melted part 41 in which the tip of the peripheral edge part 37b is melted, and an elution part 42 flowing from the tip onto the plating layer 4, and the melted part 41 and the elution part 42 are welded to the plating layer 4 in the upper surface of the plating layer 4.