Microorganism contamination prevention method for water system

A method for suppressing microorganism fouling in a water system, comprising:

• adding a reducing agent;
• and
• adding a stabilized chlorine-based oxidizing agent composed of a chlorine-based oxidizing agent and a sulfamic acid compound, with a ratio of the sulfamic acid compound to 1 mole of effective chlorine of the chlorine-based oxidizing agent being 1 to 1.5 moles, and/or a stabilized bromine-based oxidizing agent composed of a bromine-based oxidizing agent and a sulfamic acid compound, with a ratio of the sulfamic acid compound to 1 mole of effective chlorine conversion amount of the bromine-based oxidizing agent being 1 to 1.5 moles to the water system that contains the reducing agent, characterized in that
• a concentration of the reducing agent in the water system is 0.01 mg/L-Cl2 or more to 5 mg/L-Cl2 or less, and
• an addition concentration of the stabilized chlorine-based oxidizing agent and/or the stabilized bromine-based oxidizing agent relative to a concentration of the reducing agent in the water system is 2.5 times to 10 times in molar ratio.