The concentration of an isothiazoline compound used as an antibacterial agent in an aqueous system is suppressed even if the addition concentration is low, and the reduction of the concentration can be improved after 24 hours. Provide a suppression method. An isothiazoline compound is supplied to an aqueous system at an addition concentration of 0.2 mg / L or more and less than 1 mg / L, and a combined chlorine agent composed of a chlorine stabilizer and a chlorine-based oxidant is 0.3 mg / concentration in terms of Cl2. A method for suppressing concentration reduction of an isothiazoline compound, which is added so as to be maintained at L or more and 6 mg / L or less. [Selection figure] None【課題】水系において、抗菌剤として用いられるイソチアゾリン化合物が、低い添加濃度であっても、濃度の低減が抑制され、２４時間経過後における残留率の向上を図ることができる、イソチアゾリン化合物の濃度低減抑制方法を提供する。【解決手段】水系に、イソチアゾリン化合物を０．２ｍｇ／Ｌ以上１ｍｇ／Ｌ未満の添加濃度で供給し、塩素安定化剤及び塩素系酸化剤からなる結合塩素剤をＣｌ２換算濃度で０．３ｍｇ／Ｌ以上６ｍｇ／Ｌ以下に維持されるように添加する、イソチアゾリン化合物の濃度低減抑制方法。【選択図】なし
