PURPOSE: To lessen the quantity of molding material and molding a joint component having a hollow ring section by clamping a pipe-shaped molding material extruded from an extrusion section of a material feeding device in the state of half molten by means of molds, expanding the same by means of air blown into the material and cooling and solidifying the same in the state of adhering to a cavity surface.

CONSTITUTION: Two molds 4 and 5 are opened, and a half molten molding material 6 is extruded in the state of pipe-shape from an extrusion section of a material feeding device 7. Then, compressed air is blown from an air feeding device into the molding material 6 through an air blow out hole after the extruded pipe-shaped molding material 6 is clamped with both molds 4 and 5 to be located in cavities. The molding material 6 is expanded in the cavities of both molds 4 and 5 and adhered closely to the cavity faces. In said state, the molding material 6 is cooled and solidified or cured through the molds 4 and 5, and then the molds 4 and 5 are opened and the molded material 6 having a hollow ring section 3 is removed.