a method for controlling a screw injecting apparatus having a heating cylinder and a screw slidably and rotatably received in the heating cylinder and designed to carry out a plasticizing - metering phase in which a predetermined amount of a molten material is accumulated forwardly of the screw by plasticizing and kneading a raw molding material via the screw and the heating cylinder , a waiting phase in which the screw is held inactive for a period between completion of the metering and subsequent injection of the molten material , and an injecting phase in which the molten material is injected . the method comprises the step of causing the apparatus to vibrate the molten material at a predetermined low frequency axially of the screw during the plasticizing - metering phase . by vibrating the molten material at a low frequency during the relatively long plasticizing - metering phase , the viscosity of the molten material can be reduced . further , since vibrations are generated axially of the screw , sufficient vibrations can be applied to the tip of the molten material .