rod 7 a hydraulic pipe 12 having a selector valve 11 is connected to head end ports 9 and 10 of the injection cylinders 5 and 6 a hydraulic pipe 17 having a selector valve 15 is connected to rod end ports 13 and 14 of the injection cylinders 5 and 6 a hydraulic pipe 19 having a selector valve 18 is connected to the hydraulic motor 4 the hydraulic pipe 19 and the hydraulic pipes 12 and 17 merge and are connected to a hydraulic pump 23 and a hydraulic tank 24 through a hydraulic pipe 22 having a flow control valve 20 and a relief valve 21 a hopper 25 is disposed near the proximal portion of the screw 3 to supply resin between the screw 3 and the heating cylinder 2 a heater 26 is arranged around the heating cylinder 2 to heat the resin between the screw 3 and the heating cylinder 2 a servo motor e g a pulse motor 50 and a screw rod 51 coupled thereto are threadably engaged with a female threaded member 49 fixed on the piston rod 7 when the motor 50 is rotated the screw 3 is reciprocally moved through the piston rod 7 by the threadable engagement fig6 and 7 show another embodiment showing a coupling structure between a screw rod 51 and a female threaded member 49 the female threaded member 49 shown.