A fluorescent probe which is represented by the following formula (I): (wherein, R1 and R2 represent hydrogen atom, or a substituent for trapping proton, a metal ion, or an active oxygen species, or the like; R3 represents a monovalent substituent; R4 and R5 represent hydrogen atom or a halogen atom; R6 represents hydrogen atom, an alkylcarbonyl group, or an alkylcarbonyloxymethyl group, provided that a combination of R1, R2, and R3 provides (1) substantially high electron density of the benzene ring to which said groups bind, so that the compound represented by the formula (I) is substantially no fluorescent before the trapping of proton, or the like, and (2) substantially reduced electron density of the benzene ring to which said groups bind, so that a compound after the trapping, which is derived from the compound represented by the formula (I) is substantially highly fluorescent after the trapping of proton or the like). A fluorescent probe having an excellent fluorescence property is provided.