To provide a waste water treatment apparatus which has a simple constitution and, moreover, is capable of distributing raw water uniformly or with arbitrary quantities of water to a plurality of reactors (aeration tanks).

This waste water treatment apparatus is provided with an aeration tank 1 for mixing raw water with activated sludge and performing aeration, a main supply path 7 for supplying a part or all of the raw water to the aeration tank 1, a plurality of sub supply paths 10 which are branched from the main supply path 7 and a movable weir 11 which is disposed between the main supply path 7 and the sub supply paths 10. Therein, the sub supply paths 10 are connected with the aeration tank 1 leaving a prescribed interval in the longitudinal direction of the aeration tank 1.