PURPOSE: To prevent a molding material from dwelling in an excess amount and from adhering to a screw head, by flowing back an excess amount of the molding material to the circumference of a screw when the metered molding material is injected into a mold.

CONSTITUTION: The quantity of a molding material 7 to be poured into the cavity 4a of a mold 4 is to be 5W10% more than the amount to be poured plus the amount that will remain in a nozzle and a heating cylinder tip. Then the rotation of a screw 3 is stopped, and the material 7 is injected into the cavity 4a. In this case, though part of the molding material 7 will blow back, since the metered amount is excess, the pour is sufficient and the molding material 7a remains in excess in the screw tip. Then, the screw 3 is moved forward to cause the excess amount to flow back to the thread groove of the screw. Since this back-flow occurs along the surface of the screw head 3a, the dwelt amount at the screw tip is minimized.