Abstract: A screw press for squeezing out liquid from a fibrous material including a screen cage having an inlet for the material to be pressed and an outlet for the pressed material, a pair of parallel Archimedean screws, rotatably arranged inside the screen cage, each of the Archimedean screws having a main thread extending around a shaft with a height and a pitch decreasing in the direction from the inlet towards the outlet. Around each shaft and between the turns of each main thread, an auxiliary thread is provided having a reduced height with respect to the height of the main thread, and the auxiliary thread of an Archimedean screw is arranged close to the main thread of the other Archimedean screw at least along the portion of the main thread having a lesser height.