PROBLEM TO BE SOLVED: To provide a method for measuring environmental pollutants in a homogeneous system, which is performed by a simple single-step operation, for measuring in a short time by a very simple operation, without repeating addition of and cleaning in several steps, as compared to a conventional method, using GC-MC(gas chromatography-mass spectrometry) or the like, or a method for measuring in a heterogeneous system, such as ELISA(enzyme-linked immunosorbent assay) or the like. SOLUTION: A reagent for measuring a target substance (dioxin-like compound) comprises a complex, which consists of an Ah(aryl hydrocarbon) receptor marked by terbium ions and Hsp 90 marked by rhodamine, and Arnt(aryl hydrocarbon receptor nuclear translocator). The Ah receptor and the Hsp 90 are dissociated, by the bonding of the Arnt with the Ah receptor bonded with the dioxin-like compound. The reagent has the characteristic that the terbium ions of a fluorescent substance and the rhodamine of the fluorescent substance have the relation of between a donor and an acceptor, in fluorescence resonance energy transfer.
