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 hours in 24 hours, and a treatment with a compound shown by formula (1) is carried out once or more in 30 days.【課題】レジオネラ属菌を確実に殺菌し、その増殖を抑制して、冷却水中のレジオネラ属菌数を長期間不検出レベルに維持することができ、しかも、薬剤耐性菌や薬剤耐性を持った藻類、真菌類等が繁殖しにくい開放循環冷却水系の処理方法を提供する。【解決手段】開放循環冷却水系の冷却水に対して、ハロゲン系酸化物による処理を２４時間に１時間以上行い、かつ、一般式（１）で示される化合物による処理を３０日間に１回以上行う。【化１】【選択図】なし
