PURPOSE: To drastically decrease the frequencies of backwashing the entire part of filter beds by dividing the filter beds with horizontal grid to the upper filter bed and the lower filter bed, disposing an air diffusion pipes for aeration on the grid and providing the grid with a water spraying pipes for spraying washing water downward thereunder.

CONSTITUTION: The filter bed 11 of a filter tank 10 is partitioned by the grid 14 which does not allow the passage of filter media unless descending flow of, for example, 40 to 50m/hour is imparted to the lower filter bed 13 for mainly capturing SS and the upper filter bed 12 mainly decomposing BOD. The grid 14 is provided thereon with the air diffusion pipes 19 for aeration to supply oxygen and air to living thing films sticking to the filter media of the upper filter bed 12 and to enhance the activity thereof, thereby the filter bed is sufficiently provided with the function to decompose the BOD. The lower filter bed 13 for mainly capturing the SS is increased in pressure drop by the SS in a short period and, therefore, the lower filter bed is formed by lowering its bed height and decreasing the amt. of the filter media so as to be effectively backwashed by the water spraying pipes 20 disposed under the grid 14.