PURPOSE: To make efficient removal of refuse feasible by detecting the level of raw water in a casing, and by increasing or decreasing the revolving speed of a rotary screen according to whether the detected water level is above or below the set value.

CONSTITUTION: The level of raw water is constantly detected with a water level detector 1 and the revolving speed of a driving device 3 comprising a variable speed motor is automatically increased or decreased by operation of a speed controller 4 according to whether the level of the water is above or below the set value, and detachment of refuse 08 can be adjusted thereby, allowing the level of the water to be kept roughly at constant. Thereby efficient detachment and removal of the refuse become feasible.