PROBLEM TO BE SOLVED: To provide a treatment method for an open-circulating cooling water system, which sterilizes Legionella bacteria surely, inhibits the growth, can maintain the number of the Legionella bacteria in cooling water at a level not detectable for a long period of time, and further, hardly generates propagation of drug resistance bacteria, alga having a drug resistance, true fungi, etc.SOLUTION: In cooling water of an open-circulating cooling water system, a treatment by a halogen-based oxide is carried out for one or more in 24 hours, and a treatment with poly[oxyethylene(dimethyliminio)ethylene(dimethyliminio)ethylene dichloride] is carried out once or more in 30 days.