PURPOSE: To obtain a sedimentation basin having an extremely simple structure, practically nil in trouble and having high purification capacity, by arranging a truncated cone shaped sewage falling cylinder having a receiving port having an inner diameter larger than the outer diameter of a sewage outlet.

CONSTITUTION: Sewage flowed out from several sewage outlets 5 provided to the undersize of each sewage pipe 4 is fallen in truncated cone shaped sewage falling cylinders 6 from the receiving ports provided to the uppermost parts of said sewage falling cylinders 6 and the greater part of sludge is separated from water when said sewage is slowly turned to the rising stream from the lower edges of the sewage falling cylinders 6. Sewage enters a U-shaped trough from the upper edge thereof and flowed into a water tank 2.