PURPOSE: To efficiently treat sludge containing various kind of organic substances while utilizing a ligneous fragment by extracting soil with water to prepare soil bacteria-containing water for treatment and mixing sludge to be treated with the water for treatment and diluting it, and thereafter supplying diluted sludge water to a treatment medium constituted of ligneous fragment.

CONSTITUTION: In the case of treating general sludge supplied from e.g. a sludg supply part 4A selected from among many sludge supply parts 4A-4C, firstly water of a water tank 1 for regulation is supplied to a water tank 3 for dilution through a filter 2 by a pump P. Soil in a hemp bag suspended in the tank 3 is immersed in water to extract soil bacteria into water. Then general sludge is supplied from the supply part 4A to a dilution tank 5 through the pump P and also dilution water of the tank 3 is supplied by the pump P and diluted sludge water is produced. Successively diluted sludge water is supplied to a first reaction tank 6 by the pump P and homogeneously agitated and mixed with ligneous fragments packed in the inside thereof. Activated sludge is biologically treated.