PURPOSE: To perform the separation of sludge by sedimentation with good efficiency in a vertical long tank with less floor space in which a pipe with a spiral inclined plate is attached to the central part of the tank and original water is sent up along the inclined plate from below.

CONSTITUTION: The pipe 11 provided with the inclined plates 12 and the sludge drain port 13 in a spiral form is vertically provided to the central part of the cylindrical sedimentation tank 10. And, when original water is directed into the inlet 16 above the lowest part of the inclined plate 12, it goes up spirally along the inclined plates 12, during which period sludge in the original water once sediments onto the inclined plates 12, goes gradually toward the pipe 11, flows from the sludge drain port 13 into the pipe 11, and then sediments in the sludge collection portion 17. Thus, the sedimenting separation of sludge can be performed with good efficiency in the vertical tank with less floor space in this way.