PURPOSE: To efficiently biologically treat organic substance, by mixing residual anaerobic sludge-contg. waste water having passed thrugh an anaerobic tank with aerobic sludge discharged from the first aerobic tank, and then aerobically agitating the resulting newly formed sludge in the second aerobic tank.

CONSTITUTION: Waste water is gathered in a conditioning tank 1, wherein making the properties of the liquid uniform, the addition of a nutritive source, etc. are performed at need, and then its predetermined amount is continuously supplied to an anaerobic tank 2. In the anaerobic tank 2, the waste water sent from the conditioning tank 1 is stopped and aerobically agitated by air fed therein from a blower 6. Since this aerobic agitation offers a very small amount of oxygen uniformly dissolved in the waste water for the propagation of facultative anaerobes living in the anaerobic tank 2 and the formation of anerobic sludge in which said facultative anaerobes are activated, an amount of aeration per unit for contaminating substance may be at a value below a tenth of that in the first aerobic tank 3.