To obtain the subject polymer enabling the easy measurement of a fluorescent strength to control the concentration of the polymer by labeling the polymer having one or more water-treating functions with an imidazole derivative.

This polymer is obtained by labeling a polymer having various kinds of functional groups having water-treating functions such as a scale- preventing function and an anticorrosive function with an imidazole derivative (preferably 2-phenylbenzimidazole). The labeling of the polymer is carried out e.g. by a method comprising copolymerizing a monomer having one or more functional groups having a water-treating function and having a polymerizable double bond with an imidazole derivative having a polymerizable double bond. The polymer is preferably added to a treating water in a concentration of 1-100 mg/L converted into solid content in the water. The polymer may, if necessary, be used together with one or more water-treating agents such as an anticorrosive agent and a bio-fouling preventer.