PURPOSE: To provide an injection molding method which can mold even resin apt to be oxidized at a high quality and inexpensively.
CONSTITUTION: Molten resin plasticized by a plasticizing process plasticizing the resin material to be supplied from a hopper 20 in a course sending the resin in front from the hopper 20 by a turning action of a screw 3 is injected to a cavity of a mold by driving the screw 3 in an axial direction. Then, when a molded product is obtained by an injection/dwell process performing dwelling, the inside of a screw cylinder 2 is made into an inert gas atmosphere, a space between the screw cylinder 2 which is more backward than the hopper 20 and a screw shaft 6 is sealed by a sealing device 10 and molding is performed by opening a sliding shutter between the screw cylinder 2 and hopper 20 at the time of the plasticizing process and interrupting at the time of the other process.