PURPOSE: To increase an antifoaming effect and to reduce consumption by mixing beforehand the liquid mixed with activated sludge in an aeration tank with an antifoaming agent to be injected into the aeration tank in order to antifoam the froth generated on the liquid surface in said tank thereby diluting the antifoaming agent.

CONSTITUTION: Non-diluted night soil is introduced by a pump 1 into an aerating tank 3. After the night soil receives a purifying effect by activated sludge together with aeration, the night soil is admitted together with the activated sludge through a line 9 into a settling tank 4. The froth 8 generated during aeration in the tank 3 is antifoamed by the antifoaming agent fed therein through a line 11 by a pump 6 for antifoaming agent. A supply line 12 for liquid mixed with activated sludge is connected to said injecting system for the antifoaming agent, and the mixed liquid is supplied by a pump 7 into the line 11; therefore, the antifoaming agent is diluted and extended by the above-described mixed liquid and is then injected to the froth layer. Since the antifoaming effect by the injection is obtained, the antifoaming effect is increased.