PURPOSE: To improve the clarity of separated liquids and decreases the water content of separated solid phase, while the consumption of flocculation promoters is saved, by throwing flocculation promoters into a sedimentation section from a plurality of positions disposed in an axial direction, spaced suitable distance from each other.

CONSTITUTION: Sludge 27 and flocculation promoters 24 are supplied into a cylindrical sedimentation section 14a rotating about an axis through a sludge feed port 22, sludge guide plate 28 and sludge throw-in port 26, and through a chemical injection port 21 respectively. By the centrifugal forces of the section 14a, the sludge is separated into a solid phase 33 and a liquid phase 34 so that the solid phase 33 is discharged from an end 16 of the section 14a, while the liquid phase 34 form the other end 18 thereof. And chemical injection ports 21, through which flocculation promoters are thrown in, are provided at a plurality of positions spaced appropriate distance from each other in an axial direction of the section 14a. As a result, the reduction in consumption of flocculation promoters, improvement in clarity of separated liquids, and decrease in water content of separated solid phase can be achieved.