PURPOSE: To stabilize a retreating operation of a screw and to heighten the measurement accuracy of resin by a method in which the change of the back pressure of the screw adjacent to the finish of the plasticizing process of resin by open loop control, is preliminarily detected, and the standard screw-back pressure adjacent to the finish of plasticizing process is obtained, whereby the screw is retracted according to said standard screw-back pressure.

CONSTITUTION: While the screw 10 in a heating cylinder 12 is retracted, the resin with which a mold is to be filled is plasticized and measured. Then, the change of the back pressure of the screw adjacent to the finish of resin- plasticizing process is preliminarily detected, and the standard back pressure of the screw adjacent to the finish of plasticizing process is obtained, and then said back pressure is set in a controlling device 70. Next, the back pressure of the screw in each measurement cycle is detected by a back pressure-detector 50, and back pressure-controlling part 76 controls the setting pressure of a releaf valve 42 so that the detected back pressure becomes standard back pressure.