To provide a diagnostic drug which binds specifically to an amyloid aggregate and/or an amyloid deposit, to thereby realize imaging and quantification of a disease caused by amyloid aggregation and/or deposition. The invention provides a compound represented by formula (1) : (wherein X 1 represents an optionally substituted bicyclic heterocyclic group； X 2 represents a hydrogen atom, a halogen atom, or a chelate-forming group； ring A represents a benzene ring or a pyridine ring； and ring B represents an optionally substituted 5-membered aromatic heterocyclic group which is bonded to the benzene ring or the pyridine ring via a carbon atom of ring B), a salt thereof, a solvate of any of these, or a transition metal coordination compound of any of these, and a diagnostic, preventive, or therapeutic drug containing the same.
