A microbicidal composition contains isothiazolone compounds, which contain 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one, a nitrate in a content of 3% by weight to 25% by weight with respect to a total weight of the composition, a haloacetamide in a content of 5 ppm to 500 ppm, and water. A nitrate in a proportion of 3% by weight to 25% by weight and a haloacetamide in a proportion of 5 ppm to 500 ppm are added to an aqueous solution of isothiazolone compounds, which contain 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one.