Generate high concentration of hypochlorous continuously by electrolysis of electrolyte such as diluted hydrochloric acid or salt+hydrochloric acid in w/o diaphragm electrolyzer, then, generated chlorine(Cl2) at anode and dissolve chlorine (Cl2) in water. Dilute it into the feed water for generating available chlorine 10 - 80 ppm of hypochlorite.