PURPOSE: To treat waste water as it is without diluting the same by providing a control means for controlling not only the supply amount of org. waste water to the upper part of an open type container corresponding to the output of the the humidity detector provided in the air permeable packed bed of the container but also the supply amount of air to the lower part of the container corresponding to the output of a temp. detector.

CONSTITUTION: Waste water is supplied to the packed beds L1, L2 composed of the fine pieces of a plant constituting body in a container from nozzles N1, N2 to be penetrated thereinto while air is sent in the container from the bottom part thereof through an introducing pipe D to be passed through the packed beds L1, L2 from below to above. As a result, the white rot or filamentous fungi in the packed beds are propagated and the temps. of the packed beds rise by the heat generated when an org. substance is decomposed. The humidities and temps. of the packed beds are detected by the humidity detecters W1, W2 and temp. detectors T1, T2 provided in the packed beds, and the supply amount of waste water and the feed-in amount of air are respectively controlled by an automatic control apparatus C so as to hold humidities and temps. to predetermined ranges and the steady treatment of waste water is performed over a long period of time.