PURPOSE: To avoid a clogging phenomenon of a filter surface by a method wherein a water drainage cylinder, a filter cylinder and an outer cylinder having a common rotary shaft are provided and a treating liquid is introduced into a gap between the filter cylinder and the outer cylinder so as to be moved to a direction of the filter cylinder to act a centrifugal force on mixed-in substances adhered to a peripheral surface of the filter.

CONSTITUTION: If the water drainage cylinder 20, the outer cylinder 37, the filter cylinder 29 and stirring blades 34, 40 are rotated by starting a motor, a centrifugal force is acted as well as air is mixed in to produce a large amount of fine air bubbles. By said centrifugal forde, mixed-in substances in a treating liquid sent into a main body 1 from intake ports 18, 19 are blown off toward an inner wall of the main body 1 and a separation work is further enhanced. Further, when a sewage or the like is passed between each peripheral wall plates 29a of the filter cylinder 29, the filtering is carried out. Then, clarified water is raised an inner pipe of a double pipe 22 to be reached to a clarified water take-out box 25 and, further, introduced to a water storage tank.