To provide a sewage treatment system capable of effectively utilizing a water level detector installed to a sewer pipe for the purpose of flood detection for the purposes other than a measure against guerilla heavy rain, and performing effective water treatment such as an energy-saving operation.

A sewage treatment system is for making sewage water generated from a prescribed region flow in through an inflow gate 15 by a sewer pipe 13 communicated to a sewage treatment plant 12, executing prescribed treatment and discharging it, a water level detector 21 is provided on prescribed parts A1, A2 and A3 on a pipeline of the sewer pipe 13, and a water level of the sewage water flowing through the sewer pipe 13 is detected. An operation controller 18 inputs the water level detected by the water level detector 21, makes the sewage water be stored inside the sewer pipe 13 by closing the inflow gate 15 when the water level before a preset time zone is equal to or lower than a scheduled operation water level, and makes a water treatment operation be executed by opening the inflow gate 15 when the time zone starts.