To provide a method of keeping the concentration of chemical agent, capable of disinfecting sewage or sewage at the time of rainy weather, a disinfection method for a water to be treated and an apparatus for these methods.

In the method of keeping the concentration of chemical agent, a part of an aqueous solution of hypochlorite or hypobromite stored in a chemical agent storage tank is drawn out to be electrolytically treated to enhance the concentration of hypochlorite or hypobromite and the treated solution is returned to the storage tank to hold the concentration of the chemical agent in the storage tank to a definite value or more. In the disinfection method for water to be treated, a part of the aqueous solution is drawn out to be electrolytically treated to enhance the concentration of hypochlorite or hypobromite and the chemical agent is added to water to be treated and disinfected. The apparatus for this method has the chemical agent storage tank for storing the aqueous solution of hypochlorite or hypobromite, a means for drawing out a part of the chemical agent of the chemical agent storage tank, an electrolytic device for enhancing the concentration of the chemical agent of the chemical agent storage tank and a means for returning the chemical agent enhanced in its concentration to the chemical agent storage tank.