A protrusion 31 formed at an end edge of a board that is not screw-fixed is inserted into a rectangular opening engaging section 25 formed on a chassis 2, sides of the board are mounted on a screw-fixing section (also serving as a board receiver) 21, and a first and a second board receiving sections 22, 26 that are formed on the chassis 2 by a cutting and raising manner, respectively, the board 3 is gripped by board gripping sections 23a, 23b, and one side of the board 3 is screw-fixed to the screw-fixing section (also serving as the board receiver) 21.