To provide a method for controlling the back pressure of a screw press which prevents any occurrence of co-rotation phenomenon of a dehydration cake.

A back pressure plate device 9 to apply the pressure in the direction opposite to the sewage feeding direction as the back pressure is provided on a discharge port 7 of a filter cylinder 1, and the back pressure plate device 9 comprises a back pressure plate 10 and a pneumatic cylinder device 11 to press the back pressure plate 10 toward the discharge port 7 side. A pressure regulator 22 to regulate the pressure of air to be fed from a compressor 21 to the cylinder device 11, and a control device 23 are arranged. The control device 23 fluctuates the back pressure given by the back pressure plate 10 to the reference back pressure and the predetermined low level back pressure lower than the reference back pressure alternately in a time series manner by controlling the pressure regulator 22.