Electrolysis is carried out while supplying an aqueous sodium chloride solution having a weak acidic property to the anode chamber of a water electrolytic cell. The electrolytic cell is partitioned with a cation-exchange membrane into an anode chamber and a cathode chamber. Furthermore, because the pH of the anolyte is sufficiently reduced due to the acid content and pH of the liquid supplied to the anode chamber from the start of electrolysis, which is different from a conventional electrolytic method, the electrolysis of the present invention may be carried out for a time needed to obtain a sufficiently high oxidation reduction potential. Thus, acid water almost the same as that obtained in a conventional method is obtained in the present invention by consuming about {fraction (1/10)} the electric power that is used in a conventional method.