PURPOSE: To bring raw water into contact uniformly with the surfaces of respective porous tubes in a bundled body wherein the opening ends of the tubes deposited with dynamic membranes on the outside surfaces are molded by removing part of the tubes and forming a raw water introducing hole.

CONSTITUTION: Many porous tubes (a)... which are closed at one end are made flush in opening parts 2 and are inserted into a cylindrical jig (I), by which the tubes are bundled like a column. After the bundled tubes are tied at one end, the tubes are removed from the jig (I) and are tied at the other end. Of such tube bundle, some pieces located in the central part are removed to from a through- hole (III). On the other hand, a metallic ring 4 is placed on a metallic plate (IV), and a molten resin (V) is cast therein and the tube bundle is suspended and is dipped therein in such a way as to avoid the intrusion of the resin in the tubes, whereby the tube bundle is molded 5 with resin. The through-hole (III) is closed with the resin at one end and forms a introducing hole 6 for raw water. The raw water enters the tubes through the hole 6 and flows among the tubes uniformly, whereby the raw water is filtered efficiently.