To manufacture a bottomed, cylindrical operation knob having no weld line and retaining a uniform mechanical strength by injection molding in a rational manner.

A cavity-side die 1 having recessions 11a and 11b for molding the outer peripheral surface and front end face of an operation knob, cylindrical core dies 3a and 3b having a runner 15a and a restriction gate 16a, and a runner 15b and a restriction gate 16b, respectively, and a platelike stripper plate 2 having circular holes 2a and 2b which allow front-end sections of the core dies 3a and 3b to penetrate therethrough so that the front-end sections are fitted tightly therein, respectively, are used to inject a heated plastic melted material into cavities 12a and 12b and to cool it to solidify. Subsequently, the stripper plate 2 and the core dies 3a and 3b are separated from the cavity-side die 1 in a die opening process. The core dies 3a and 3b are then pulled out of the circular holes 2a and 2b of the stripper plate 2, and products (12a) and (12b) are removed from the dies 3a and 3b.