PURPOSE: To enhance filtering efficiency, by providing a closed circuit containing a parallel flow type final filter using a precise filter membrane to a final stage.

CONSTITUTION: Raw water 1 is heated by a heater 2 and preliminarily filtered by a submerged type filter 3 and receives secondary filtering by a membrane filter 4 with a pore size of 10μm. Subsequently, chemicals 5 are injected in filtered water by a pump 6 while the treated water is sent to a reverse osmosis module 8 by a pump 7. The water passed through the reverse osmosis membrane passes through a cation exchange resin tower 10 and an anion exchange resin tower 11 to be stored in a water storage tank 12 and subsequently passed through a mixed bed tower 14 of a cation exchange resin and an anion exchange resin by a pump 13 and always recirculated through a closed circuit again returning to the water storage tank 12 from a filter 15 by a membrane filter with a pore size of 1μ through an ultraviolet sterilizing lamp 16 and a final filter 17. Particles 28 are impinged against a membrane 21 at an incident angle θ.