PURPOSE: To control the backpressure of a screw responsively and accurately by electrically measuring the pressure of a melted material or the backward force of the screw as the backpressure of the screw.

CONSTITUTION: The backward force applied on the screw 1 due to the transfer of material generates a turning effort on the screw shaft in the backward direction of the screw through the screw moving member 6 and the screw bearing member 10. At the same time, the screw backpressure control unit 22 is driven to apply a braking force on the screw shaft 11. As a result, the backpressure of the screw is applied on the melted material and the weight of the material is measured. In this case, as the reaction of the backpressure, a thrust force generates on the shaft 11. The backpressure is supported by the wall part 4b through the thrust bearing 12 so that stress is applied to the wall part 46 throughit is small, occurs at a place with a strain gage 21 in proportion to the backpressure, the amount of deformation is measured electrically by a strain gage 21 and detected as the backpressure. Accordingly, this unit is not influenced by the frictional force of a transfer mechanism, etc., it is excellent in responsiveness and can control the backpressure control with a high precision.