PURPOSE: To provide the control system, which can follow up time variation in inflow sewage and compensate control delay due to the operation time of a simple discharge gate, for the gate provided at the run-off ditch of the primary settling basin of a confluent inflow type sewage treatment plant.

CONSTITUTION: The pumping flow rate to the primary settling basin 4, the run-off ditch water level, the quantity of deviation between the run-off ditch water level and a target water level, the difference between the run-off ditch water level and a downstream water level, and the frequency of gate opening and closure are regarded as input variables and an opening and a closing command to the gate are inputted to a fuzzy inference part 11 to perform gate control by fuzzy inference.