A compound represented by the formula (I) or a salt thereof (R1 represents hydrogen atom, or a substituent; R2 to R7 represent hydrogen atom, hydroxyl group, an alkyl group, or a halogen atom; R8 and R9 represent hydrogen atom, or an alkyl group; X represents a C1-C3 alkylene group; and R10 represents an acyl group), which promptly causes a structural change from a non-fluorescent closed ring structure to a strongly fluorescent open ring structure by hydrolysis of an acyl group, and is useful as a fluorescent probe for measuring a protease showing superior suitability for quantification.