PURPOSE: To suppress the solidification of scales without conducting a high- voltage reverse electrolytic cleaning by holding the internal residual water as such when the inflow of the water to be electrolyzed is stopped, inverting the polarity of the voltage to be impressed on an electrode at the same time and lowering the voltage.

CONSTITUTION: The water to be electrolyzed is detected by a water passage detector 1, purified in a water purifying part 2 and supplied to the lower part of an electrolytic cell 4 through a check valve 3. The cell 4 is divided by a semipermeable diaphragm 5 into the anode 6 side and cathode 7 side, and an electrolyzing voltage is impressed on both electrodes from an electrolyzing power source 8. Meanwhile, an acidic water discharge passage 9 is formed in the anode side of the cell 4 at a position slightly higher than an alkaline water intake passage 10, and accordingly the upper part of the cell 4 is inclined. When the stoppage of water passage is detected by the detector 1, the polarity of the electrolyzing voltage is inverted by a control part 11, and an extremely low voltage is impressed on both electrodes 6 and 7 so that a feeble current flows between both electrodes 6 and 7 with the water remaining in the cell 4 as a medium.