A method for fluorescently labeling a protein is provided that can also be used for labeling a biomolecule (a protein) in vivo with fluorescence. The method for fluorescently labeling a protein includes obtaining a fusion protein of a target protein to be labeled and, for example, a photoactive yellow protein (PYP), and fluorescently labeling the target protein by reacting the fusion protein with a compound represented by Formula (I), wherein in the compound represented by Formula (I), a coumarin skeleton in Formula (A) or a benzene skeleton in Formula (B) is intramolecularly associated with a group X and as a result, fluorescence derived from the group X is suppressed and when in the compound of Formula (I), the coumarin skeleton in Formula (A) or the benzene skeleton in Formula (B) is bound to the PYP or the PYP-derived protein contained in the fusion protein, the coumarin skeleton in Formula (A) or the benzene skeleton in Formula (B) is intramolecularly dissociated from the group X, and thereby fluorescence derived from the group X can be emitted.
