It is intended to stabilize an aqueous solution of isothiazolone compounds, which contain 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one. A nitrate in a proportion of 3% by weight to 25% by weight and a haloacetamide in a proportion of 5ppm to 500ppm are added to the aqueous solution of the isothiazolone compounds, which contain 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one.