A sintering compound containing a sinterable powder and a binder removable in a debinding step is injected into a metal mold set provided with a sintering compound injecting mold, in a sintering compound molding step (S104). An add-on forming compound which becomes removable or separable from the sintering compound injection molded body in the debinding step or the sintering step is injected into the metal mold set provided with an add-on mold in an add-on forming step (S102). The sintering compound injection molded body and the add-on are formed integrally with each other in the metal mold set. The integral formation of the add-on and the sintering compound injection molded body enables to prevent damage during release from the mold and make easy to handle the injection molded body.