To prevent leakage of plastic plasticized without unevenness of an injection amount by rotatably arranging a plasticizing screw in a plasticizing cylinder, and slidably assembling a rotatable injection plunger having an end shape of a screw with an axial center of the screw.

A rear end of an injection plunger 4 is mounted at a rotary shaft 17 contained in a crosshead 16. The crosshead 16 is coupled to a piston head 21. Further, the head 21 slides in a hydraulic cylinder 22 of a driver case 12. This rod 21 is driven to forwafd or reverse the plunger 4. A plasticizing cylinder holder 7 and driver case 12 are coupled and fixed on a frame 24. Accordingly, the plunger 4 is rotated at twice or more rotating speed as fast as a plasticizing screw 2 in the same direction by an injection plunger driving wheel 29 via a movable key 30 and slid in an axial direction.