Wobble caused between an operation knob and a casing for housing the operation knob is suppressed as small as possible due to the heat contraction at the time of a molding process. As a result, the operation knob can be downsized and so an assembly of the operation knob and the casing can be miniaturized as compared with the conventional ones. The operation knob and the casing for guiding the movement of the operation knob are formed by composite resins by using a dichroic molding method, and then the operation knob and the casing are separated after being molded to obtain an assembly. Due to the heat contraction at the time of the molding process, a necessary guide clearance is formed between the operation knob and the casing after the assembling of the operation knob and the casing. The operation knob is allowed to move due to the guide clearance.