A first groove, a second groove, and a prepared hole are formed in a traverse chassis. The first groove restricts the downward movement of a printed circuit board as a lower edge portion of the board is slidably pinched in the first groove. The second groove restricts the sliding movement in one direction of the board as an edge portion on one side of both lateral sides of the board whose lower edge portion is nipped in the first groove is fitted in the second groove. The prepared hole vertically penetrates a portion of the traverse chassis defining the first groove and is disposed adjacent to an edge portion on another side of the both lateral sides of the board whose edge portion on the one side is fitted in the second groove.