PURPOSE: To maintain the pH of circulating water for cleaning electrodes in a prescribed range and to prevent the clogging by CaSO4, the scaling and the corrosion of a device by supplying the pH control water of the circuit which is independent from a circulating water path.

CONSTITUTION: The pH value of waste water is detected by a pH controller 27 and the driving of a caustic soda soln. pump 11 and a pH control water pump 21 is controlled by the detected pH value. A pH controller 28 detects the pH value of the circulating water in a circulating water tank 2 and controls the driving of the pump 11 and the pump 21 by the detected pH value. The pH value of the circulating water is controlled to ≤9 at which CaSO4 does not deposit and the pH value of the waste water is controlled to ≥4 at which corrosion is not generated. Since the pH control water path is provided independently from the circulating water path in the above-mentioned manner, the pH of the circulating water is surely controlled by controlling the pH values of the waste water and the circulating water in the prescribed range.