PURPOSE: To save the labor and power of machinery and to reduce control load by connecting a water supply pipe to the double pipe suspended to the vicinity of the bottom plate of a tank in a branched state and performing filtering and washing by the same water supply pipe by changing over a valve.

CONSTITUTION: The water supply pipe 2 connected to the emitting port of a water supply pump 1 to be extended therefrom is branched and the branch pipes are respectively connected to the outer pipe 4 and inner pipe 5 of the double pipe 3 suspended into a cylindrical filter tank 8 so that the lower end part thereof approaches the protrusion having a trigonal pyramid shape provided to the bottom plate of the tank 8. A valve 7 is provided to the branch pipe 6 connected to the outer pipe 4 and, when filtering treatment is always performed, the valve 7 is opened to supply water to the outer and inner pipes 4, 5. When the granular filter material of a filter material bed 10 is periodically washed, the valve 7 is closed to supply water only to the inner pipe 5 and the emitting force of the inner pipe 5 is raised to move the granular filter material around the protrusion 9 to stirr the same. The removal of impurities and the backwashing treatment of the filter material are easily performed by changing over one water supply pump.