The first electrode section (7a) and the second electrode section (7b) are formed such that the outer portion connected to the upper portion of the heating section (6) is thinner than the inner portion connected to the lower portion of the heating section (6). The heating section (6) and the first electrode section (7a) are interconnected by an R-shaped first connecting portion (21). The inner circumferential sloping surface of the heating section (6) and the second electrode section (7b) are interconnected by an R-shaped second connecting portion (22). The first and second connecting portions (21)(22) are formed such that R becomes larger from the upper end to the lower end, where the upper portion is thin and the lower portion is thick. The thickness of an intermediate portion (6b) is smaller than the thicknesses of the first and second connecting portions (21)(22).