PURPOSE: To reduce dehydration time and electric power consumption, by adding an additive having high hydrating power beforehand to a material to be dehydrated, dehydrating said material by an electrosmosis method, and, if necessary, by recovering said additive by using a reverse osmosis membrane.

CONSTITUTION: An electrode 2 is provided to the second half of a dehyrator 1 such as a belt press, and electric power is applied thereto from an electric power source device 2. After sludge 6 is mixed with flocculating agents 7 and additives 8 having high hydrating power, the sludge is supplied to a dehydrator 1 and is thus dehydrated. Dehydrated cake 12 is discharged to the outside of the system and separated filtrate 9 is passed through a reverse osmosis membrane 4, whereby the above-described additives 10 are recovered and reused. If the filtrate 9 is pressurized by using the membrane 4, the additives are recovered as they are, and it is only necessary to make up the additives by as much as the additives contained in the cake 12 and discharged. The consumption of the additives is decreased considerably.