To improve mounting workability of a mounting board, as well as to prevent the mounting board from slip off caused by excessively loosening a screw.

The screw 14 having a head part 14a is screwed with a female screw part 10a through a spring washer 12, a plain washer 16 curved on a side of the head part 14a of the screw 14, and a circular hole part 15b of a potbelly hole 15a of a board 15 to be mounted, a long hole part 15c of the potbelly hole 15a of the board 15 is inserted between the plain washer 16 and the mounting board 10 to tighten the screw 14, and the board 15 is detachably fixed to the mounting board 10.