Abstract: The electrode holding portion is formed of band metal bent round into an annular shape, and a pair of lugs are formed at the ends of the holding portion. At the free end of one lug, a stopper is formed by being squarely bent toward the facing lug. Additionally, beside the flanges formed protruding outward from the top rim ranging from the electrode holding portion to the lugs and also along the bottom rim of the lug, reinforcement pieces are formed along the top and bottom rims of the stopper continuously respectively ranging from the flanges. Since deformation of the stopper is prevented by the reinforcement pieces, the deformation of the electrode holding portion due to excessive diametric reduction is surely prevented.