A fluorescent probe which is represented by the following formula (I): (wherein, R1 represents a monovalent substituent other than hydrogen atom, carboxy group, or sulfo group; R2 represents hydrogen atom, or a monovalent substituent; R3 and R4 each independently represents hydrogen atom or a halogen atom; and R5 represents a monovalent group which is cleaved by contact with a measuring object, provided that a combination of R1 and R2 is selected so that the oxidation potential of the benzene ring to which they bind makes (1) the compound represented by the formula (I) substantially no fluorescent before the cleavage, and (2) a compound after the cleavage, which is derived from the compound represented by the formula (I), substantially highly fluorescent after the cleavage).