To improve the decomposition treating efficiency of sewage.

The biological contact filter media 19 comprise main contact filter media 20, sub-contact filter medial 21 and connecting members 22. The main contact filter media 20 are bellows pipes which are opened at both ends and are formed to an approximately cylindrical shape. The sub-contact filter medial 21 which are the bellows pipes which are opened at both ends and are formed to the approximately cylindrical shape are disposed in the space parts 20b of the main contact filter media 20. The sub-contact filter medial 21 are supported in the space parts 20b of the main contact filter media 20 by the connecting members 22 in the state that the peripheral wall parts 21a of the sub-contact filter medial 21 are parted from the peripheral wall parts 20a of the main contact filter media 20. According to this constitution, the air and sewage are sufficiently supplied to the microorganisms by surely increasing the surface areas to which the microorganisms stick, by which the decomposition treating efficiency of the sewage is improved.