To realize highly efficient, highly capable, continuous and stable process for the removal of water content from a thermoplastic resin stock, by a method wherein conveying and pressure-rising screws, the screw pitches of which gradually become smaller from their upstream side toward its downstream side, and compressing screw elements for compressing fed stock are provided in dehydrating parts.

A dehydrating part 16 consists of a pressure rising part 16A and a compressing part 16B. In the pressure rising part 16A, screw pitches are made gradually smaller from its upstream side toward its downstream side. Concretely, from the uppermost-stream side of the pressure rising part 16A downwards, a large pitch screw part 16A1, a medium pitch screw part 16A2 and a small pitch screw part 16A3 are provided. The material passed through the pressure rising part 16A is strongly compressed at the passing through the sealing ring of the compressing part 16B, resulting in flowing the separated water backwards so as to discharge from the dehydrating screen part 7 of a dehydrating barrel 8. In addition, the water content left in the material is separated simultaneously kneading at the kneading part 17B of a devolatilizing and kneading screw part 17 so as to be discharged through a devolatilizing part 17A from the vacuum vent parts 13 of vent barrels 9.