To improve cleaning efficiency of a hollow fiber filter, by equipping a cleaning promoting means to rub the inside face of a cylindrical hollow fiber filter which purifies fluid by passing from the inside to the outside and cleans the filter by passing opposite.

During operation of a circulation hot water bath, bath water in a bathtub 1 is purified by sucking the water through a suction port 2 and a water supply hose 3 by the action of a circulation pump 6 and passing the water from the inside to the outside through the hollow fiber filter 7a in a purifying tub 7. Highly viscous soil or biological membrane adhered on the inside of the hollow fiber filter 7a is rubbed off by a flow member 16 flowed by the flow of bath water. On the contrary in back wash operation, the bath water delivered from the circulation pump 6 is passed from a switch valve 9a through a back wash passage, a switch valve 9c from the outside of the hollow fiber filter 7a to the inside to peel off the soil, etc., adhered on the inside and discharged outside together with pumped water through a switch valve 9b and a drain passage 11.