PURPOSE: To prolong the service life of the filter medium on the whole by providing an inner cylinder in a filter vessel, furnishing an inlet for water to be treated at the upper part of the side wall of the filter vessel, and allowing the water to be treated to flow down through the downcommer part formed by the side wall and the inner cylinder.

CONSTITUTION: The liq. to be treated flows in from the inlet 6 for the liq. to be treated, the dynamic pressure is relaxed by the outer surface of the inner cylinder 22, and the flow is straightened while the liq. flows down through the annularly formed downcommer part 25. The liq. is then reversed into a filter chamber 4 from the lower end of the inner cylinder 22, sent upward, and passed uniformly through each hollow yarn membrane structure 9. As a result, solid matter is filtered and separated on the surface of the hollow yarn membrane 10, the treated liq. is sent upward through the hollow part of the hollow yarn membrane 10, passed through the water collecting pipe of a hollow yarn membrane module 11, and discharged from a treated liq. outlet 19 via a treated liq. chamber 5. When the filter is backwashed, air is injected from the nozzles 16 of an air pipeline 17 to vibrate the hollow yarn membrane 10, and the clogging components are removed.