To treat oil-containing drainage, whose treatment has been difficult, easily and effectively by activated sludge by cultivating a group of mixed microorganisms in cutting oil drainage integrally and adding the group when drainage is introduced.

In order to prepare an activated liquid (sludge) to be added before sub-anaerobic treatment, a group of mixed microorganisms, after being cultivated integrally from oil-containing drainage and propagated, is added into sludge to prepare activated sludge for drainage treatment. Next, drainage to be treated is introduced into a treatment tank, and the activated liquid is added further into the tank as required. Subsequently, it is maintained in a sub-anaerobic state and only agitated to prevent separation into the drainage and the sludge. In the sub-anaerobic treatment, DO(dissolved oxygen) is not 0 mg/liter in general but kept extremely low. After the sub-anaerobic treatment, aerobic treatment is done so that aerobic conditions in which DO is about to be about 1.5 mg/liter on average are brought. After the aerobic treatment being done, anaerobic treatment is done for about 0.5-2 hr.