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.