To prevent a leakage current from flowing and lessen power loss by providing an insulator with a shallow fold made at the upper part of the outer surface and a deep fold made under this insulator fold.

An ordinary insulator fold 40 is provided at the upper part of an insulator 5, and a deep fold 41 is made at the center of the insulator 5. The insulator fold 40 is made so that the sectional form may be shaped like a mushroom with a sooth curve in vertical direction from the outermost periphery, with an outside diameter D1, being narrowed down at the lower part of the outermost periphery. The deep fold 41 is made so that the diameter may be larger in downward oblique direction, and the lower side is made in the shape of a cylinder with an outside diameter of D2, and the lower end side is made in the shape larger in outside diameter than D2. A leakage current can be prevented from flowing to lessen power loss, by providing the insulator fold and the deep fold made under this insulator fold this way.