The invention provides a bactericidal/algicidal method including adding an oxidant-based bactericidal/algicidal agent and a stabilizer therefor to a target water system, characterized in that the amount of combined chlorine or the stabilizer in the water system is controlled by generating free residual chlorine in the water system, and a bactericidal/algicidal method including adding an oxidant-based bactericidal/algicidal agent and a stabilizer therefor to a target water system, characterized in that the amount of the oxidant-based bactericidal/algicidal agent added is controlled so that the concentration of total residual chlorine in the water system falls within a predetermined range, and the amount of combined chlorine or the stabilizer is controlled so that the concentration of free residual chlorine in the water system falls within a predetermined range. An object of the present invention is to provide a bactericidal/algicidal method including adding an oxidant-based bactericidal/algicidal agent and a stabilizer therefor to a target water system for bactericidal/algicidal treatment, wherein the amount of combined chlorine or the stabilizer is controlled, and the amount of the oxidant-based bactericidal/algicidal agent added is controlled, which method realizes effective utilization of the stabilizer, to thereby reduce the amount of the stabilizer employed and to reduce COD and the amount of nitrogen derived from the stabilizer, and which method does not require a special control apparatus for in-line mixing as described above.
