When a stationary metallic mold and a movable metallic mold are closed, a housing is molded having a retainer insertion hole by the existence of a sliding mold. A retainer is also molded. First, when only the metallic mold is opened, the core pin is withdrawn, and the sliding mold is withdrawn in the longitudinal direction by the cam action of the angular pin, so that the molding portion is withdrawn from the retainer insertion hole. Successively, the sliding mold is relatively drawn out to the front of the advancing and withdrawing passage together with the stationary metallic mold, and the advancing and withdrawing passage becomes open. Next, an auxiliary sliding mold is advanced, and the retainer held by the auxiliary sliding mold is inserted into the retainer insertion hole and temporarily assembled to the housing. It is possible to insert the retainer straight into the retainer insertion hole. The finished product is protruded to the front of the movable metallic mold by the protruding pin.