PURPOSE: To reduce the water content of dehydrated cake and to improve the speed of dehydration, in the dehydration of sludge, by preadding incineration ash to the sludge and then adding a macromolecular flocculant to it to form coagulated flocks.

CONSTITUTION: Sludge to be dehydrated is supplied to a mixing tank 1 through a pipe 2 at its bottom, while a predetermined amount of the incineration ash of the dehydrated cake of sludge is concurrently added through a hopper 3, and sufficiently agitated and mixed with the sludge by an agitator 4. The overflowing mixture is supplied through a pipe 9 to a mixing tank 5, while a predetermined amount of a macromolecular flocculant is concurrently added through a supply pipe 6 at the bottom, agitated and mixed with the sludge by an agitator 7 to flocculate the sludge. The sludge is then supplied to a dehydrator. Since the incineration ash is inorganic substance, the strength of the flocks formed by mixing it with the organic sludge is so large that the mixture can be dehydrated into dehydrated cake having a small water-retaining ratio at a high dehydration speed without permeation of water content through the belt of filter cloth in dehydration using a belt press or the like.