PURPOSE: To quickly manufacture the titled hollow body having desired length, by piling up a plurality of filter plate units with each other while abutting adjacent facing flanges on each other, and connecting both of the outside filter plate units in a state of tension by a coupling rod.

CONSTITUTION: The hollow body A for the formation of biomembranes having a coupling rod 13 whose both end parts are conected to the arm of a rotary shaft horizontally pivoted in a filthy water tank is used as a means for mixing air in filthy water or the like. That is, with the rotation of the rotary shaft, filthy water flows into each reaction chamber 12b, 12b through the notched parts 8a, 9a of the hollow body A for the formation of biomembranes facing the surface of filthy water just before being introduced from an upper part above the surface of filthy water into filthy water. The filthy water is purified by the repeated inflow and outflow of air and filthy water into and from each reaction chamber 12b, 12b. The hollow body for the formation of biomembranes is made to predetermined length only by adjusting the number of standardized filter plate units.