To provide a waste water treatment facility wherein increase in sludge generated in waste water treatment plant is suppressed and the amt. of disposal of sludge is decreased.

In a waste water treatment facility wherein org. substances included in waste water are decomposed with microorganisms included in sludge and are made to flow out as a treated water, a first aeration tank 2 utilizing propagation of the microorganisms and a second aeration tank 3 utilizing internal respiration of the microorganisms are installed and mixed liq. floating substance meters 5 are respectively set in each tank and each tank is alternately switched by means of switch valves 6 based on increase and decrease in concn. of floating substances in a mixed liq. to suppress increase in excess sludge and to decrease the amt. of disposal of the sludge.