PURPOSE: To reduce fluctuation of the amount of resin filled into a mold by advancing a screw in a state wherein a valve nozzle is previously closed before injection after weighing is completed and raising the pressure of the front part of an injection cylinder and then opening the valve nozzle to start injection and advancing the screw at the prescribed amount from a point of this time.

CONSTITUTION: When a screw 4 is slightly advanced in a state wherein a valve nozzle 6 is closed from the state such as molten resin is weighed and accumulated in an injection cylinder 3 of the tip of the screw 4, the resin weighed and accumulated in the tip is allowed to reversely flow to the direction shown in an arrow from a passage 81 of resin. A counter-flow preventive ring 8 closes the passage 81 in the advance position t1 and the pressure of resin is suddenly changed. A controlling device 14 controls an injection molding machine 1 so that the valve nozzle 6 is allowed to abut against the inflow port 20 of a mold 19 at a point of time when the screw 4 reaches the position t1. When the valve nozzle 6 is allowed to abut against the inflow port 20, the valve 61 is opened and the resin weight and accumulated in the tip part of the injection cylinder 3 is injected into the mold 19. When the position of the screw 4 reaches the prescribed position measured by a screw position detector 10 together with advance, the controlling device 14 is stopped in the advance and stoppage position t3.