PURPOSE: To contrive an accurate scraping-off of filter cake by a method wherein a number of immovable divided scraper pieces are attached to the inner surface of a spiral screw provided on the outside of a cylindrical filter body with their ends overlapped and held by the force of a spring into a slidable contact with the outer peripheral surface of the filter.

CONSTITUTION: A rotary outer cylinder mechanism 3 is provided around and coaxially of a fixed cylindrical filter body 22 having a cylindrical outer peripheral filter surface 22F provided with an open filter hole 21Ac of a desired size. A spiral screw 41 whose outer end is fixed to the rotary outer cylinder mechanism 3 is so arranged as to permit its free rotation with its inner wall surface out of contact with the outer peripheral filter surface 22F. A number of arcuate spring-loaded scraper pieces 51 into which a scraper mechanism is divided are provided in the grooves 41M of the inner wall surface and fixed in their respective positions and held immovable in the groove direction by projecting parts 41T and biased by a spring 52 to a normal position wherein the inner peripheral surfaces of the divided scraper pieces 51 are placed in a slidable contact with the outer peripheral filter surface 22F. As a result, the filter cake can be scraped off the outer peripheral surface of the filter uniformly in its entirety.