To easily, safely and effectively prevent red water and to maintain the effect over a long period by subjecting an iron product, in which red rust is present or may be present, to treatment with polyphenol-contg. water.

As the iron product to be applied, optional water feeding piping made of iron in which, particularly, iron rust has been generated on the surface at the inside is listed. As the water feeding piping, the one whose inside has been galvanized is also usable. Preferably, polyphenol-contg. water and these iron products are brought into contact for at least 4 to 6 hr. Moreover, the contact temp. is not particularly limited, but, since it is preferably at 20 to 30°C, namely, in the vicinity of room temp., the execution of the treatment is also easy. Polyphenol to be used can easily be obtd. from various plants, and is, e.g. extracted by using water from the leaf of tea. Its concn. in the polyphenol-contg. water is suitably controlled to about 0.1 to 0.5%, and its pH is preferably controlled to 4 to 8.