PURPOSE: To inhibit overaeration, by arranging a fixed water level-detecting level switch in the aerating precipitating vessel of a batch-type active sludge treating apparatus so that its detection signal can be transmitted through the first timer to the second timer for switching a blower into an intermittent operation state if the water level becomes lower than a predetermined one.

CONSTITUTION: When the water level in an aerating precipitating vessel 1 comes into the state of a waste water level L-3 at which a blower 2 shall not be continuously driven to inhibit the overaeration of waste water in the vessel 1, the blower is automatically switched into an intermittent operation state by a mechanism involving a level switch (LS) 10 to detect the level and a couple of timers 11, 12, one (11) of which is driven by a signal from the level switch 10 and the other (12) of which is driven by a signal from the timer 11.