To uniformly and efficiently purify circulating water and the whole passage of the circulating water without using a harmful halogenated compound in a system for circulating the circulating water.

In the system comprising a plurality of units capable of treating water, and piping 5 connecting these units, and circulating the circulating water 3, an enzyme group extracted from the fruit of an angiosperm, such as a pineapple and the like, is added to the circulating water to inhibit contamination of the units, the piping 5, and the circulating water 3. In this method, the enzyme group may be added from a chemical liquid feeder 7 to the circulating water 3 at 1-5,000 ppm in total. Additionally a nonionic surfactant may be added to the circulating water. A mass ratio of the total amount of the enzyme group to the nonionic surfactant is, for example, about 1,000/1 to 1/1. The system may be an open system or a closed system. The system may be provided with a refrigeration unit 2.