PURPOSE: To prevent the breakage of a cathode conductive belt even though a diaphragm is repeatedly pressed in the treatment and to improve the durability of the belt by laying the belt free from a sludge treating vessel.

CONSTITUTION: The cathode conductive belt 6 is laid with respect to the sludge treating vessel 1 so as to make it slidable at the pullout parts piercing the vessel, and freely displaced in the vessel when pressing the diaphragm 5 by providing a tension pulley 19 at the midway of the belt to exert a fixed tension on the belt. Consequently, when the belt is pressed from its back when pressing the diaphragm in the dehydration stage, the belt is not expanded or deformed, and the belt itself is pulled into the vessel in accordance with the expansion of the diaphragm. Accordingly, excessive internal stress is not exerted on the belt, and the belt is not broken.