PURPOSE: To provide an injection molding machine wherein the longitudinal length of its screw can be shortened to reduce its size and an increase in the installation space and in the molding machine costs can be held down.

CONSTITUTION: The injection molding machine 1 is composed of a cylinder 2, a screw 3 arranged therein and a screw driving device 4. The screw body 5A of the screw 3 is gradually increased in diameter from its rear end part 3a to the front end part 3b thereof. A deaeration port 10 is provided on the outer periphery of a cylindrical part 7b of a hopper 7 located on the side of a material supply source from the rear end part 2a of the cylinder 2 and a pressure reducing pump 12 is connected to the deaeration port 10 via a pipe l1. Therefore, a gaseous component is produced as a high-molecular material is plasticized, separated therefrom, allowed to flow toward the rear end part 3a of the screw 3 oppositely to the flow of the high-molecular material and discharged from the deaeration port 10 through the hopper to the outside. For this reason, there is no need for providing a stepped part, thereby shortening the longitudinal length L1 of the screw 3.