PURPOSE: To lower the water content of a cake while accelerating the treating speed of sludge, by a method wherein a polyamide type cationic org. high-molecular flocculant is added to sludge and, thereafter, a specific anionic org. high- molecular flocculant is added thereto to perform mechanical dehydration treatment.

CONSTITUTION: After a polyamide type cationic org. high-molecular flocculant is added to and mixed in sludge, an anionic org. high-molecular flocculant comprising partial hydrolysate of sodium polyacrylate or polyacrylamide is added to and mixed in said sludge to perform flocculation treatment and the treated sludge is subjected to mechanical dehydration treatment. In this case, the polyamide type cationic org. high-molecular flocculant is polyamide polyamine prepared by reacting alkylenediamine represented by formula H2N-R-NH2 (wherein R is a 2W6C straight or branched chain alkylene group), epihalohidrin and basic polyamide obtained by reacting polyalkylene polyamine having two primary amino groups and at least one secondary or tertiary amino group in the molecule thereof and aliphatic dicarboxylic acid or lactam thereof in a predetermined ratio.