PURPOSE: To reduce running cost, the size of a device, etc. by increasing the metabolic product by the action of bacteria as far as possible in a sludge culturing stage and mixing and charging activated sludge-like material and raw waste water in a biological reaction stage.

CONSTITUTION: The soln. mixture contg. the activated material formed and activated in a sludge culturing stage 1 and raw waste water contg. an org. material are mixedly charged into a biological reaction stage 2, so that the polluting components in the waste water are made into the state permitting a solid-liquid sepn. Part of the soln. mixture is introduced into an active oxidation stage 3 where the above-described separating performance is much improved and thereafter the soln. is fed into a sepn. stage 4 so that the soln. is separated to sludge and waste water. The remaining soln. mixture is thereafter fed to the stage 1 where the sludge-like material formed by the action of bacteria and the metabolic product contg. oxidized enzyme, etc. are increased as far as possible and after the sludge and the sludge-like material are made surface active, the sludge and the sludge-like material are returned again to the biological reaction stage.