PURPOSE: To backwash valves of respective openings successively and effectively with a small amount of washing fluid by providing a slider to open the valves of respective openings successively movable in the longitudinal direction at the time of moving on the periphery of a washing fluid feeding pipe with a number of openings incorporated in a cylindrical filter.

CONSTITUTION: A number of openings and check valves 22 are disposed on a washing fluid feeding pipe 21, and a backwash device 20 with a slider 23 incorporated slidably in the up and down direction is incorporated into a ceramic cylindrical filter 12 of a filter device 10. When a filter 12 is backwashed, compressed air for washing is fed from an inlet port 15a, while compressed air for moving the slider 23 is fed from an inlet port 15b respectively. The pipe 21 is in the closed state in the initial stage, and the slider 23 is lifted up to actuate respective check valves 22 and open the same selectively, guiding the compressed air in the pipe 21 to the inner periphery of the filter 12 locally and successively to purify the filter 12.