In a cooling water system having been subjected to an antimicrobial treatment using chlorosulfamic acid or a salt thereof, algae growing therein are effectively killed and removed. An antimicrobial and algicidal method for a cooling water system characterized in that in a cooling water system subjected to an antimicrobial treatment using a chlorosulfamic acid and/or a chlorosulfamic acid salt, when algae are generated, an isothiazolone-based compound is further added to the cooling water system. The chlorosulfamic acid and/or a salt thereof are added to the cooling water system in such a manner as to give a residual chlorine concentration of 0.5 mg-Cl 2 /L or higher.
