PURPOSE: To perform stable oxidation treatment by packing fluid media of ≤1 specific gravity which flow downward in a flowing tank.

CONSTITUTION: When air is fed into an air dissolving pipe 2, waste water is inevitably sucked from the bottom end of the pipe 2 by an air lift effect and while it ascends, the air is dissolved into the waste water. At the same time, the waste water is given the velocity of downward flow in a flowing bed part 5. In the bed 5, fluid media 4 of ≤1 specific gravity stuck with aerobic microbes are kept floated in the upper part of the flowing bed, are given the velocity of downward flow; at the same time, they are flowed downward in the flowing bed part and the lower interface of the fluid media is formed upper than the bottom end of the pipe 2. In this method, the always stable oxidation treatment is accomplished without clogging by the aerobic microbes.