It is an object of the present invention to provide a fluorescent protein, which allows an acceptor for fluorescence resonance energy transfer (FRET) to appear in a stimulating light-dependent manner, thereby enabling the marking of any given cell organelle, cells, or tissues, with multiple colors. The present invention provides a fluorescent protein which consists of a fused protein of a donor fluorescent protein and an acceptor fluorescent protein, wherein before irradiation with stimulating light, the donor protein is able to emit fluorescence as a result of irradiation of the donor protein with excitation light; and after irradiation with stimulating light, intramolecular FRET occurs between the donor fluorescent protein and the acceptor fluorescent protein as a result of irradiation of the donor protein with excitation light, and the acceptor protein is able to emit fluorescence, and wherein the fluorescence of the donor protein and the fluorescence of the acceptor protein have wavelengths that are different from each other.