PURPOSE: To perform the biological treatment of waste water with a high efficiency and to reduce the operation expense, by flowing the waste water through the packing material decreased or increased the packing quantity in flowing direction of the waste water successively.

CONSTITUTION: The contact oxidation vessel 3 is divided in plural sections so as to make the volumes of the packing materials 41, 42, 43 in order of each large, middle and small and these are arranged isolating by the rectifying boards 15, 16 in order of downward flowing direction of the waste water 1. The gas dispersing apparatus 9W11 are provided to the upper part of each material 41W43 and oxygen necessary for aeration, is jetted. The water 1 is flowed from the inflow opening 2 in the contact oxidation vessel 3 and is flowed from the outflow opening 5 contacting with the materials 41W43 successively and performing aeration. On occasion of generating filamentous fungus easily, the order of flow of the water 1 or arrangement of the materials 41W43, is reversed. By this treating method, regulation of aerobic bacteria in the vessel 3 is sufficiently performed and decomposition of BOD or nitrification of ammonium ion etc., is carried out stably.