A compound represented by the following formula (I): ##STR00001## wherein R.sup.1 and R.sup.2 represent amino groups that substitute at adjacent positions on the phenyl ring, provided that either of R.sup.1 and R.sup.2 represents a mono(C.sub.1-6 alkyl)-substituted amino group and the other represents an unsubstituted amino group; and R.sup.3 and R.sup.4 independently represent hydrogen atom or an acyl group, and an agent for measurement of nitrogen monoxide which comprises said compound.