Heat exchange water for cooling an object of heat exchange such as machinery, air, or liquid, which serves to prevent oxidation and deterioration of metal materials used in pipes for supplying/circulating the heat exchange water or in the liquid ends of the heat exchanger, to suppress growth of algae and microorganisms, and to reduce influence on the environment. The heat exchange water is reductive water having zero or negative standard oxidation-reduction potential as determined on the basis of the hydrogen electrode standard.