To automate the process extending from the start-up to shutdown by maintaining the minimum supply of water to form a screenings roll for separating screenings from sewage for a definite time and then gradually increasing the supply to the supply to form the screenings roll in normal operation.

The raw water stored in a raw water tank G is supplied to a measuring tank 1 by a pump P, and the measured sewage is supplied to a rotary-screen filter R. Namely, in the initial stage of operation, the minimum measured water necessary to form a screenings roll is supplied to the filter R to form an initial screenings roll on the rotary screen, the supply of sewage is then gradually increased in appropriate amts. to increase the amt. of rolled screenings, and stabilized operation is attained at the maximum capacity. The sewage discharged for supplying the measured water is returned to the raw water storage tank G. The filtered sewage freed from screenings is supplied to a biological treating plant.