PURPOSE: To treat waste water by a biological filtration method excellent in the removal of BOD and SS.

CONSTITUTION: In waste water treatment method by a biological filtration wherein water to be treated is passed through a carrier bed in an immersed state as a descending stream and air is diffused to the lower part of the carrier bed to raise air bubbles to purify water to be treated by aerobic biological decomposition and filtering action, a biological bed is formed using granular matter as a carrier 2 and divided into two upper and lower sections (R-zone and F-zone) and air diffusion members 3, 5 emitting air are respectively provided to the boundary part of both sections and the lower part of the lower section. In a usual treatment process, water 9 to be treated is allowed to flow in the upper section from above and air is diffused from the air diffusion member 3 to mainly perform the decomposition of org. matter and the removal of particles in the upper section and the removal of particles is mainly performed in the lower section to discharge treated water 10 from the lower part of the lower section and, in washing process, air is also diffused from the air diffusion member 5 or 3 and washing water is sent in the lower part of the lower section to discharge waste washing water 12 from the upper part of the upper section.