PURPOSE: To exactly inject a fixed quantity of plastic material in a mold by a method wherein a slant for moving a back-flow prevention ring backward is provided at the tip of a screw shaft and on the back-flow prevention ring and the back-flow prevention ring is moved backward by rotating the screw shaft in reverse direction so as to shut off a circuit, through which the plastic material flows backward.

CONSTITUTION: After the predetermined quantity of plastic material is stored in a space 28 or the metering process is completed, a screw shaft 12 is rotated in reverse direction a little before the injection process is started. The engagement between the side face C of a projection 38a and the step D of a screw head 36 is released so as to slide the slant B of the screw head 36 along the slant A of the projection 38a in order to gradually move a back-flow prevention ring 38 backward until the rear end face E of the back-flow prevention ring 38 is pressed against the front end face F of a screw shaft main body 32. Consequently, the passage before and after the back-flow prevention ring 38 are put into a completely disconnected state. Thus, because injection is performed under a state of completely preventing the plastic material in the space 28 from back-flowing, a precisely fixed quantity of the plastic material can be injected.