A compound represented by the formula (I) wherein R1 and R2 represent amino groups that substitute at adjacent positions on the benzene ring, wherein one of the amino groups may have one alkyl group R3 and R4 represent hydrogen atom, a C1-6 alkyl group, or a C1-6 alkoxy group, R5 and R8 independently represent a C1-6 alkyl group, R6 and R9 represent a carboxy-substituted C1-6 alkyl group, an alkoxycarbonyl-substituted C1-6 alkyl group, a sulfo-substituted C1-6 alkyl group, or an alkyl sulfonate-substituted C1-6 alkyl group, and R7 and R10 represent a C1-6 alkyl group, an aryl group, a C1-6 alkoxycarbonyl group, a vinyl group, a thienyl group, or a pyrrolyl group, and an agent for measurement of nitrogen monoxide containing the compound. Said compound can give a fluorescent substance that is free from change in fluorescence intensity in a wide pH range.
