PURPOSE: To obtain dehydrated cake having low water content at a low cost, while stabilizing dehydrability, by adding a ferrous salt type flocculant and an alkali agent to organic sludge, oxidizing ferrous ion, and then thickening and/or dehydrating the sludge.

CONSTITUTION: Ferrous sulfate 2 is added to organic sludge 1, and the sludge is let flow into a sludge-reforming vessel 3. In the sludge-reforming vessel 3, an oxidizing agent such as chlorine or oxygen-contg. gas 4 such as air is supplied, and pH of slurry in the vessel is maintained near a neutral level by the addition of an alkali agent 5. After the organic sludge 1 stays in the reforming vessel 3 for a fixed time, it is separated into dehydrated cake 7 and a dehydrating effluent 8 in a sludge-dehydrating step 6. A sludge-thickening step such as a centrifugal thickener may be provided at the preceding stage of the sludge- dehydrating step 6, to dehydrate the thickened sludge. A macromolecular flocculant may be added to an effluent from the vessel 3 to enhance a solid-liquid separating speed in the thickening-dehydrating step.