PURPOSE: To provide an aerobic treatment method for organic drainage which can reduce or eliminate the generation of excessive sludge and treat stably the drainage keeping the amount of aerobic microorganisms constant without lowering load and treatment efficiency.

CONSTITUTION: In an aerobic treatment system 1, liquid to be treated 14 is introduced in an aeration tank 11, into which sponges 13 are put to carry active sludge, and mixed with the sponges 13, returned sludge 15, ozone-treated sludge 24, and suspended sludge in the tank to be treated aerobically. In an ozone treatment system 2 for making biomass soluble, the extracted sludge 22 from the aeration tank 11 is circulated to an ozone treatment tank 21; the sludge, after being contacted with ozone and decomposed by oxidation, is returned to the aeration tank 11.