Abstract: The present invention provides a fluorescent substance excellent in thermal resistance, which can be uniformly dispersed into plastic materials, and has a high fluorescence intensity even after experienced heat history in plastic forming processes. More specifically, the present invention provides a fluorescent substance containing a rare earth complex having benzophenone or benzoyl substituted with an alkyl group, a cycloalkyl group, an acyl group or an alkoxy group having a carbon number of 1 to 20 as a skeletal structure, in which a plurality of rare earth ions are coordinated with one or more types of molecules having a photosensitizing function.