To find optimum purifying interval depending on the progress rate of clogging by detecting the position of an inside float changed correspondingly to the increase of negative pressure at the suction side of a pump, which is related to the clogging state of a filter.

The negative pressure of the suction side of the pump 33 for sucking water through a water collecting pipe 15 inserted in the inside of a filter cartridge 13 is increased when the filter cartridge is clogged. Then the water surface 10c in a pipe line 36b of a detecting pipe 36, one end part of which is communicated with the suction side of the pump 33 through an extending pipe 37 and another end part of which is opened above the surface of water, becomes lower than the water surface 10a of the outside of the detecting pipe 36. When the inside float 38 floating on the water surface 10c reaches a prescribed position deeper than the water surface 10a of outside of the detecting pipe 36 by the lowering of the water surface 10c in the pipe line 36b, a control means works to detect the position by a sensor 39 and to wash the filter cartridge 13 by a washing means 34.