The present invention provides a method for suppressing a decrease in the concentration of an isothiazoline compound, wherein even if an addition concentration of the isothiazoline compound used as an antibacterial agent is low in a water system, the decrease in the concentration thereof can be suppressed so that the residual rate thereof after 24 hours can be raised. The method is carried out in a manner wherein to a water system, an isothiazoline compound is fed with an addition concentration of 0.2 mg/L or more and less than 1 mg/L, and a combined chlorine agent consisting of a chlorine stabilizer and a chlorine-based oxidant is added so as to maintain a concentration thereof at 0.3 mg/L as Cl2 or more and 6 mg/L as Cl2 or less therein.