To obtain a fluorescent labeling agent with which the concentration and the application quantity of the coating solution can simply, exactly and promptly be measured by adding a small amount of a specific thiourea polymer having high fluorescence intensity.

A polymeric thiourea having strong fluorescence intensity, for example, a compound of formula III (m and n are each an integer) prepared by reaction of a polyallylamine of formula I (n is an integer) with fluorescein isothiocyanate of formula II or a compound of formula V (m and n are each an integer) prepared by reaction of fluorescein isothiocyanate of formula II with polyethyleneimine of formula IV (n is an integer) is used as a fluorescent labeling agent.