A piston rod includes a large-diameter portion that is formed on the side of a movable contact, and a small-diameter portion that is formed on the side of an insulating operation rod relative to the large-diameter portion, and that is formed with an outer diameter smaller than the inner diameter of an inner peripheral surface of an insulating cylindrical member. The insulating cylindrical member includes a cylindrical portion that is interposed between the outer peripheral surface of the small-diameter portion and the inner peripheral surface of the insulating operation rod, a bottom portion that is arranged opposed to the distal end of the small-diameter portion, and a through hole through which a coupling pin is inserted through the piston rod and the insulating operation rod.