PURPOSE: To reduce the electric energy consumption of the subject dehydrator and to control the significant deviation of the pH of dehydrated cake from that of a raw slurry by supplying an alkaline soln. when the potential of the particles suspeneded in the slurry is negative, and supplying an acidic soln. when the ξ potential is positive.

CONSTITUTION: A couple of electrodes 2 and 6 are opposed with a slurry passage 3 in between, and a voltage is impressed between both electrodes. The slurry 10 supplied to the passage 3 is electro-osmotically dehydrated under such conditions. An alkaline soln. is supplied from the side of the electrode having polarity opposite to that of the ξ potential (electrokinetic potential) of the particles suspended in the slurry 10 toward the slurry in contact with the electrode by soln. supply means 12W16 when the ξ potential is negative, and an alkaline soln. is supplied from the outside when the ξ potential is positive. As a result, the electric energy required for the electroosmotic dehydration of excess sludge, etc., can be reduced, and the significant deviation of the pH of the dehydrated cake from that of the raw slurry can be controlled.