An electrolyzer for producing alkaline and/or acidic water by way of electrolysis of water. In order to remove scale such as calcium carbonate deposited on the electrodes of the electrolytic cell (36) during electrolysis, the control unit (166) operates a polarity reversal switch (186) at a predetermined timing to reverse the electric potential applied to the electrodes of the electrolytic cell (36). The control unit (166) includes means (190/192) for detecting the hardness of water and varies the duration of application of DC voltage of opposite polarity in accordance with the hardness of water. Hardness of water is preferably determined by detecting the electric conductivity of water. Time required for removal of scale is shortened.