Side plates, constituting side surfaces of a speaker cabinet, each comprise a base member having a plurality of grooves formed in its outer surface (obverse surface) region at predetermined intervals, and a decorative plate adhesively bonded to the outer surface of the base member, in a laminated fashion, so as to cover the grooves. The laminated base member and decorative plate are pressed and curved, via upper and lower molds, so as to provide a smooth, convexlycurved outer surface. The grooves are adhesively bonded along their opening edges to the inner surface of the decorative plate, so that the opening edges of each of the grooves will not move toward or away from each other when external force is applied to the side plate.