To detect and display a water level according to the pressure of air which is discharged into water from an air discharge part.
The air fed from a control unit 13 to an air discharge part 11 through an air tube 12 is discharged into the sewage in a sewage tank 30 from the air discharge part 12. In the control unit 13, a switch mechanism which operates according to the pressure of the air discharged from the air discharge part 12 is provided, and an underwater pump arranged in the sewage tank 30 is controlled by the switch mechanism. The pressure in the air tube 12 is supplied to a pressure sensor 17 and detected by the pressure sensor 17. The output of the pressure sensor 17 is supplied to a digital water level display unit 18 and the level of the sewage in the sewage tank 30 is displayed on a water level display unit 18.
KASAMATSU KAZUHIDE