A fluorescent probe which is represented by the following formula (wherein R1 and R2 represent hydrogen atom, or a substituent for trapping proton, a metal ion, or an active oxygen species; R3 represents a monovalent substituent other than hydrogen atom, carboxy group, or sulfo group; R4 and R5 represent hydrogen atom, a halogen atom, or an alkyl group; R6 to R9 represent an alkyl group; R10 and R11 represent hydrogen atom, a halogen atom, or an alkyl group; M− represents a counter ion; and the combination of R1, R2, and R3 (1) imparts a substantially high electron density to the benzene ring to which they bond so that the compound can be substantially non-fluorescent before trapping proton, a metal ion, or an active oxygen species, and (2) substantially reduces electron density of the benzene ring to which they bond so that the compound after trapping proton, a metal ion, or an active oxygen species can be highly fluorescent after the trapping.