To suppressed water-bloom or suspended solid in a pond water and to adsorb and decompose a soluble sludge on a filter media while stagnating water pollutants by the action of microorganism generated on the filter medium by arranging fine mesh sheets above and below the filter media of zeolite having a specific particle diameter.

The fine mesh sheet is spread on the whole surface in a filtration tank and the zeolite having 10-50mm particle diameter is placed thereon as the filter media 5. By feeding the pond water in which coarse refuse is removed by a mesh screen 1 to a biological filtration tank 2 and bringing into contact with the surface of the filter media 5, the generation of the microorganisms on the surface of the zeolite of the filter media is accelerated and the water pollutants are received by the mesh sheet 4, the water-bloom or the suspended solid in the pond water is suppressed while obstructing the flow-down by the action of the microorganisms generated on the zeolite and particularly the soluble water pollutants are excellently adsorbed on the surface of the zeolite on the filter media 5 and decomposed.