There is provided an injection molding apparatus of the screw in-line type which has a screw having the following structural features and is used for injection-molding a high-nitryl resin.(1) A compression ratio is set to a value of 1.8 to 2.5.(2) Ratios of the lengths of the feed section, the compression section, and the metering section are set to[1 to 1.5]:[1.8 to 2.8]:[1].(3) A depth T (unit:mm) of groove of the metering section in relation to an inner diameter D (unit:mm) of the cylinder is set toT.gtoreq.0.057D-0.2.(4) A radius of curvature R (unit:mm) of the arc surface of the rear side corner portion which is formed between a screw shaft and a flight in relation to the inner diameter D (unit:mm) of the cylinder is set toR.gtoreq.0.36D+2.0.