PURPOSE: To decompose easily and completely the sludge to be treated by keeping the mixed layer of a treating medium and the sludge to be treated within a prescribed range with a water-content regulating means and a temp. regulating means, and aerating intermittently with an aerating means.

CONSTITUTION: A fan 24 is intermittently operated at specified intervals of 5W 6min/hr, and the air, about 1/10 times the volume of a treating vessel 1 per minute, is supplied into the treating vessel 1 to aerate intermittently a treating medium and the sludge to be treated. Consequently, adaptative bacteria corresponding to fat, protein and other organic materials are generated and propagated in the layer of the treating medium which is also used as a culture medium for microorganisms, and the biochemical treatments of the sludge such as decomposition, nitration, and fermentation due to the activity of the microorganisms are carried out. Any treatment residue can not be found in the treating vessel 1, and the treating medium can be semipermanently used without the need for maintenance and replenishment.