PURPOSE: To provide a screw-driving device that can be made compact and is capable of bearing strong reaction force produced by the screw for a preplasticating type injection molding machine.

CONSTITUTION: On occasion of plastication, the rotation of a motor 5 is transmitted directly to a screw 1 by operation of a piston member 35. In transmission of the rotation, reaction force from the screw is borne by a thrust bearing 50 that is arranged in an oil-hydraulic chamber 49, while radial load of the piston member 35 is borne by bushes 45, 46. On occasion of injection, oil pressure is supplied to the oil-hydraulic chamber 49, and the screw is moved forward by the piston member and thereby, a pasage communicated with an injection chamber is shut off. At this time, the thrust bearing 50 is immersed in the oil supplied to the oil-hydraulic chamber 49 and the sliding surfaces of the bushes 45, 46 are also soaked in the same oil.