To efficiently dehydrate a sludge difficult to dehydrate by adding an inorg. flocculant to a sludge, then adding an amphoteric org. high molecular flocculant, gravity-dehydrating the flocculated sludge, then adding an inorg. flocculant and centrifugally dehydrating the flocculated sludge while injecting an amphoteric org. high molecular flocculant into a centrifugal dehydrator.

An inorg. flocculant is added to a sludge (raw sludge) to be treated in a first reaction tank 1 and agitated, hence the charge of the sludge is neutralized, the sludge is conditioned, and soluble phosphorus is fixed. The conditioned sludge is then supplied to a second reaction tank 2, an amphoteric polymer is added and agitated, hence the sludge is granulated and flocculated, and the granulated and flocculated sludge is sent to a gravity dehydrator 3 and dehydrated. An inorg. flocculant is again added and strongly agitated in a third reaction tank 4, and the charge of the sludge is neutralized to a higher extent. Subsequently, the sludge in the third reaction tank 4 is supplied to a centrifugal dehydrator 6 by a pump 5, and an amphoteric polymer is injected into the dehydrator to dehydrate the sludge.