To improve characteristics of filtration treated water direct after back washing by intermittently performing the back washing, that is, by provid ing still standing time.

When back washing a filter 24, a zeta-potential adjusting agent is added into back washing water from a flocculant storage tank 30. A quantity of the zeta-potential adjusting agent to be added is controlled particularly by zeta-potential of filter media of the filter 24 which is measured by a zeta- potential meter 38. As a result, the quantity of the zeta-potential adjusting agent to be stuck to a surface of the filter media by the back washing is properly controlled, capturing effect of the flocculation floc is improved and the good quality filtered water is obtained even direct after the back washing.