To provide a method where, though, in the water electrolyzing cell of a water electrolyzing apparatus in which hydrogen and oxygen are generated, ions moving through an electrolytic membrane from an anode side by long-term continuous operation and metallic ions eluted by the deterioration of the metal of a feed conductor contaminate a membrane electrode joined body consisting of an electrolytic membrane and a porous feed conductor on the cathode, the contamination of the membrane electrode joined body caused by these ions is prevented, and the increase of electrolytic voltage and the reduction of electrolytic efficiency are prevented.

At the time when electrolytic voltage is increased to a prescribed value or higher by the long-term continuous operation of a water hydrolyzing apparatus, acid cleaning for a water circulation flow passage on the cathode side is performed, and the circulation water is replaced with ion exchange water from a pure water feed device, and the pH of the circulation water is returned to the neutral one. By this method, impurities in the membrane electrode joined body can be removed, and stable electrolytic properties can be maintained.