PURPOSE: To facilitate maintenance by opposing an insulator mounting plate to the side surface equipped with the connection terminal of a dust collecting element and arranging the same to a main body casing in a slidable manner.

CONSTITUTION: When a dust collecting element 7 and an insulator mounting plate 16 are cleaned because of contamination, the dust collecting element 7 is allowed to slide along rails 14, 14 and drawn near to an inspection service port 12. Since a wide service space is formed in a main body casing 8 by taking out the dust collecting element 7, hands are inserted in said space to make it possible to easily perform work for detaching the end parts of conductors 21, 21 from the respective terminals of relay insulators 18. Continuously, the insulator mounting plate 16 is drawn out of a guide groove 15 and taken out of the inspection service port 21 to complete drawing-out work. By this method, maintenance can surely be performed.