To reduce power consumption while stably holding treatment function.

A sewage treatment apparatus 1 is equipped with an underwater aerator 5 mixing air supplied from a blower 7 with sewage to stirr the sewage, a load measuring device 23 measuring the load of the underwater aerator 5 and a controller 21 raising the output of the aerator 5 when the measured load is less than a predetermined value. Since the output (frequency) of the underwater aerator is raised to forcibly discharge and remove entangled air when the entanglement of air is generated in the underwater aerator 5 and load becomes low, it is unnecessary to excessively set the output of the underground aerator 5, and, therefore, there is no wastefulness in power consumption and the shear fracture of activated sludge can be prevented and treatment function can be kept stable.