An object of the present invention is to provide a labeling agent which exhibits excellent binding to a target molecule in a bioassay or histological staining method that relates to a compound containing a nitrogen-containing aromatic ring. The labeling agent according to the present invention is a labeling agent having a structure in which a molecular target drug, which is a compound containing a nitrogen atom-containing aromatic ring, is bound with a label via a divalent linking group, wherein one end of the divalent linking group is bound to a carbon atom of the nitrogen atom-containing aromatic ring. The label is preferably a fluorescent substance-integrated nanoparticle, and the nitrogen atom-containing aromatic ring is preferably a pyridine ring.
