To enable easy and secure welding by welding bonding faces of projection parts provided along openings of both ends of an insulating pipe and projection parts provided along the peripheral edge of an insulating plate.

A metal case 1 is covered with an insulating pipe 3 made of polyethylene, where projection parts 2 are formed along inner perimeter of both ends of the insulating pipe 3. The two openings of the insulating pipe 3 are closed, where insulating plates 4 made of polyethylene are fitted. Projection parts 2 are formed along the outer perimeters of the insulating plates 4, and both projection parts 2 are bonded together through the insulating plate 4. And both projection parts are welded at the bonding face by heating externally. For welding, a jig having a shape matched with the curve of the projection parts are prepared, and with the heated surface pressed against the outside of both projections, it is moved toward circumference direction of the projection part to heat. As a result, only bond area can be heated effectively, and generation of non-fused area, pinhole, crack, etc., can be prevented.