PURPOSE: To prevent the damage to the molding device and to improve the quality of the final product.

CONSTITUTION: Grooved are formed on the surface of the circular cylindrical self-consumption type plunger formed of a molding material. This self- consumption type plunger is inserted in a pressurized state into a heating cylinder 12. The self-consumption type plunger itself is melted by advancing the self-consumption type plunger, by which the molding material in a molten state is injected from an injection nozzle 13 and is packed into a mold 17. The tendency of the molding material to deformation in the intermediate part of the self-consumption type plunger is absorbed by the grooves and, therefore, the galling of the inside wall of the heating cylinder is prevented. The tendency of the molding material to the deformation in the intermediate part of the self-consumption type plunger is relieved backward and is absorbed by forming the above-mentioned grooves as the spiral grooves formed spirally along the axial direction of the self-consumption type plunger.