PURPOSE: To pass organic sewage at high filter velocity and to purify it at a high degree by supplying and diffusing air from a diffusion part provided to a fixed bed consisting of sponge small bodies and practically constituting an equipment of the fixed bed without fluidizing the sponge small bodies.

CONSTITUTION: Organic sewage is supplied to a first fixed bed B wherein sponge small bodies A are utilized as a carrier from a supply pipe 1. The water level of organic sewage is positioned in the slightly upper part of a drain drawing pipe 11. Organic sewage flows down while it is brought into contact with the air bubbles discharged from an intermediate diffusion part 3 provided between the first fixed bed B and a second fixed bed C in a counter current. The same passes through the interval and the inside of the carrier and is brought into contact with organism films in the presence of air and BOD contained therein is decomposed. SS contained in organic sewage is simultaneously trapped. BOD and one part of SS are removed in the first fixed bed B and the purified organic sewage is introduced into the second fixed bed C and completely filtered herein. The treated water is stored in a storage tank via an outflow pipe 4.