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 that comprises, when algae grow in the cooling water system having been subjected to an antimicrobial treatment using chlorosulfamic acid and/or a salt thereof, further adding an isothiazolone-type compound 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-Cl2/L or higher.本發明有效地殺死並去除在由氯磺胺酸(鹽)進行抗菌處理的冷卻水系統中所產生的藻類。本發明為一種冷卻水系統的抗菌．殺藻方法，其特徵係：在由氯磺胺酸及/或氯磺胺酸鹽進行抗菌處理的冷卻水系統中，當藻類產生時，進一步加入異噻唑啉酮系化合物於該冷卻水系統。氯磺胺酸及/或氯磺胺酸鹽於冷卻水系統中的添加量使氯殘留濃度達0.5mg-Cl2/L以上。
