PURPOSE: To make it possible to allow variations in size of a mounting hole and to enable hole plugs to display cut-off efficiency without shakes.

CONSTITUTION: Female legs 18 and male legs 20 of both up and down hole plugs 10 are inserted into a mounting hole 12. Then, the hole plugs 10 are rotated to set positions so that the male legs 20 of one of both up and down hole plugs 10 coincide with fitting sections 22 of the female legs 18 of the other of them, and the male legs 20 are fit with the fitting sections 22 of the female legs 18. Then, claw sections 28A enter fitting holes 26. By the constitution, the same shaped hole plugs 10 are fit with each other, and discs 16 cover both up and down of the mounting hole 12.