First of all, a sub-PCB (6) is fixedly put on a surface of a chassis (5) as shown in an arrow I. At this time, a taper face (9a) of a guide rib (9) guides a fixed position of the sub-PCB (6) in a transverse direction. Next, the sub-PCB (6) is slid forward as shown in an arrow II. A front end of the sub-PCB (6) is led into a taper face (8b) of a fixed click (8) and is guided downward, and gets into the underside of the click and stops in a position of an abutting face (8c).