To provide a reagent with excellent stability under storage, which can detect a subject compound to be measured with higher specificity and sensitibity.Complexes of a compound represented by the general formula (I): ##STR1## (wherein R.sub.1 through R.sub.7 independently represent hydrogen atom, halogen atom, alkyl group, aryl group, aralkyl group, sulfonate group, amino group, styryl group, nitro group, hydroxyl group, carboxyl group, cyano group, or aryl azo group; R.sub.1 through R.sub.7 may or may not be bonded to each other to form a substituted or an unsubstituted condensed ring; R represents a divalent organic residue; and X.sub.1.sup..crclbar. represents an anion) and the like with immunoglobulin and the like are formed.