PURPOSE: To effectively dehydrate sludge changed in properties by adding ≥2 kinds of cationic org. high molecular flocculating agent having different degree of cationization in a proportion to adjust the mixture to have a cationic property suitable to cause flocculation of the sludge.

CONSTITUTION: An inorg. flocculating agent 10 is added to raw water 9 in a flocculating sedimentation tank 1 and causes flocculating sedimentation, then, the settled sludge 11 by flocculation is discharged. By introducing aerated liquid 12 into a sedimentation tank 2 for the activated sludge, one part of sludge is used as the returned sludge 13 and the residual part is discharged as excess sludge 14. And then, by mixing the sludge 11 with the sludge 14 and introducing to reaction tank 5, the mixture is flocculated by the flocculating agents which are added repsectively with prescribed ratio by pumps 15, 16 from a flocculating agent tank 6 for a high cationized slude and low eationized sludge 7. And the mixture is dehydrated by a centrifugal dehydrating machine 8 and is divided into separated water 17 and cakes 18. When the properties of the sludge change, the proportion of the flocculating agents to be added is so adjusted as to attain the most suitable cationic degree.