Provided is a method for suppressing a reduction of isothiazoline compound concentration in which, in a water system, even when an isothiazoline compound used as an antimicrobial agent is added at a low concentration, it is possible to suppress a reduction in concentration and improve the residual rate after 24 hours. A method for suppressing a reduction of isothiazoline compound concentration, in which an isothiazoline compound is fed to a water system at an addition concentration of 0.2 mg/L to less than 1 mg/L, and a binding chlorine agent comprising a chlorine stabilizer and a chlorine-based oxidizer is added so that the concentration in terms of Cl2 is maintained at 0.3 mg/L to 6 mg/L.提供一種在水系中，即使作為抗菌劑而使用的異噻唑啉化合物為低的添加濃度，亦可抑制濃度降低，實現經過24小時後的殘留率提高的抑制異噻唑啉化合物濃度降低的方法。一種抑制異噻唑啉化合物濃度降低的方法，其是在水系中，以0.2 mg/L以上、不足1 mg/L的添加濃度供給異噻唑啉化合物，且以維持為以Cl2換算濃度計為0.3 mg/L以上、6 mg/L以下的方式添加包含氯穩定劑及氯系氧化劑的結合氯劑。
