PURPOSE: To restrain the growth of microbe in a heat exchanger under high humidity condition, and prevent the heat exchanger from generation of corrosion and foreign odor by a method wherein an organic polymeric resin film containing a bactericide having dissolve retardancy to water is formed on the surface of a heat exchanger made of aluminum.

CONSTITUTION: A bactericide having dissolve retardancy to water is effective as a bactericide, for instance, 2-(4-thiazolyl)-benzimidazole and the like are available. The content of those bactericide to be contained in an organic polymeric resin film is desirable approximate 0.01W10wt%. In case that the anticorrosion is not required to regard as important and only an object for retaining effect of bactericide is required, the bactericide can be used within the range from more than 10wt% to less than 90wt%. The most suitable resin film having a water resistance and retaining effect of bactericide is a resin film 0.2W2g/m2 in coating thickness formed of a water soluble resin, a urethane resin and an epoxy resin made of a copolymer which consists of α-olefin and α, β unsaturation carboxylic acid.