PURPOSE: To improve utilization efficiency of a driving device by connecting if necessary a metering and injecting screw with the first spline shaft by means of the first transmitting device, connecting if necessary the first spline shaft with the second spline shaft by means of the second transmitting device and displacing the metering and injecting screw by means of a displacing device.

CONSTITUTION: The second gear train is constituted of a gear 76 connecting a motor 42 which is the second driving device and a spline gear 78 slidably fitted in the second spline shaft 62 and the first gear train is constituted of a gear 64 connecting a motor 44 which is the first driving device and a spline gear 68 slidably fitted in the second spline shaft 62. One end of the spline shaft 62 is connected with a ball screw 46 and another end thereof is connected with a spline shaft 60 by means of the second transmitting device. Another end of the spline shaft 60 is connected with a metering and injecting screw 52 by means of the first transmitting device. Metering is performed by means of the first driving device and a back pressure is applied by means of the second driving device and injection and dwelling are performed by means of the first and the second driving devices.