Under the condition that the stationary metallic mold and the movable metallic mold are closed, the housing is molded into a shape having the retainer insertion hole by the existence of the sliding mold. At same time, a retainer is also molded in the below portion. First, when only the rear metallic mold is opened, the core pin is withdrawn, and at the same time the sliding mold is withdrawn along the advancing and withdrawing passage in the longitudinal direction by the cam action of the angular pin, so that the molding portion is withdrawn from the retainer insertion hole.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, when the metallic mold is opened, 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, the 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. Since the entire width of the retainer is supported by the auxiliary sliding mold, 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.