PROBLEM TO BE SOLVED: To provide a marine organism adhesion prevention method capable of sustaining the effect for a long period by applying an agent with a low concentration, and yet preventing variety kinds of marine organisms from adhering.SOLUTION: The marine organism adhesion prevention method comprises performing, in a normal order, or in a reverse order, or at the same time, the steps of: causing a bonded halogen which is a reaction product of an amino group-containing compound selected from compounds with a molecular weight less than 300 having an amino group and capable of generating a zwitterion and their salts with a halogen-containing compound selected from hypochlorous acid, hypobromous acid and their salts to be present in seawater in a cooling water system by seawater so as to be 0.005-0.5 mg/L as a residual chlorine concentration and causing hydrogen peroxide to be present in the seawater in the cooling water system by seawater so as to be 0.1-2.0 mg/L, to prevent marine organisms from adhering to the cooling water system by seawater by causing the bonded halogen and hydrogen peroxide of both steps to coexist.
