PURPOSE: To prevent the adhesion of foreign substances to the internal wall surface of a bushing by a method wherein an insulator plug is pushed against the internal wall surface of the bushing at all times upon the attaching/detaching work of the insulator plug.

CONSTITUTION: An insulator plug 21, inserted into a gap 20 existing between divided internal electrode 5A and the same electrode 5B, is integrated with a pushing metal 24 through a connecting member 27. The pushing metal 24 is screwed into the penetrating hole 23 of the internal electrode 5B to push the insulator plug 21 against the internal wall of a bushing 3 with a predetermined pressure. The connecting member 27 connects the insulator plug 21 to the pushing metal 24 and, therefore, the torque of the pushing metal 24 is never delivered to the insulating plug 21. When the pushing metal 24 is pulled out, the insulating plug 21 is also pulled out integrally. The elastic pushing member 40 of a supporting shaft 33 pushes the connecting member 27 and, therefore, a condition that the insulator plug 21 is pushed against the internal wall surface of the bushing 3 can be kept from a time when the screwing of the pushing metal 24 is started.