PURPOSE: To increase easiness for use of a filter by shortening the washing time of the second filter incorporated with a hollow yarn film and preventing washing water from flowing into activated carbon of the first filter.

CONSTITUTION: A filter is provided with both the first filter 2 enclosed with activated carbon and the second filter 3 incorporated with a hollow yarn film which has numberless fine holes and is capable of precision filter. In such a case that the filter performance of both filters 2, 3 is reduced by using this filter for a long period, raw water is flowed into an outflow port 3b of the second filter 3 via a water path 8 by changing over a flow path changeover valve 5 for raw water, a flow path changeover valve 7 and a drainage port valve 11. iron rust or the like stuck on the hollow yarn film of the inside of the second filter 3 is removed by introducing raw water through an inflow port 4 for raw water and the water is discharged to the outside of the filter through a drainage port 13 via a pipeline 9, a branched pipe 10 and a drainage port valve 11.