To provide a structure for fixing a side panel without causing any damage on the exterior design at a low manufacturing cost.

Since stoppers 32, 33a and 33b are formed integrally at the rear end part 31 of a side panel 3 folded back inward, material cost and labor can be reduced and traces of spot welding are prevented from emerging to the surface of the side panel thus damaging the appearance of a product. Furthermore, a worker is protected against cutting a finger because the folded part has a hemming edge. The side panel 3 fits the stopper 33a in a stop hole 26, stops the stopper 33a at the edge 26a of the stop hole 26, pushes the stoppers 32 and 33b into the longitudinal edge (stop edge) 22b and stops them in plate, and then screws a flange 34 on a post 2. Since the flange 34 is bent at an acute angle slightly smaller than right angle with respect to the side panel 3, the side panel 3 does not inflate outward when it is screwed on the post 2.