PURPOSE: To accurately obtain dehydrated cake having a low water content with simplified operation, by adding a macromolecular flocculant to organic sludge formed from industrial waste water, etc., mechanically dehydrating the sludge, adding poly (iron sulfate) to the dehydrated cake, and then mechanically dehydrating the cake.

CONSTITUTION: After a macromolecular flocculant such as an anion or cation type is added to organic sludge formed from night soil, sewage, industrial waste water, etc., the sludge is dehydrated by a mechanical dehydrator at the first stage. After poly (iron sulfate) as a dehydrating assistant is then added to the resulting dehydrated cake at a ratio of 1W10wt% as Fe by solid matter, the cake is dehydrated into further dehydrated cake by a mechanical dehydrator at the second stage. By the addition of said poly (iron sulfate), the dehydrated cake having rigid dense structure is obtained. A belt press, a pressing dehydrator or a centrifugal dehydrator may be used as the first-stage dehydrator, while a belt press, a screw press or the like may be used as the second-stage dehydrator.