The object is to provide a reagent which comprises a compound having affinity for amyloid, and which can detect amyloid in vitro and in vivo with high sensitivity. Thus, disclosed is a reagent for detecting amyloid deposited in a biological tissue, which comprises a compound represented by the formula (1) or a salt thereof. [Chemical formula] (1) wherein A1, A2, A3 and A4 independently represent a carbon or a nitrogen; R1 represents a group selected from the group consisting of a hydrogen, a hydroxy group, a carboxyl group, a sulfate group, an amino group, a nitro group, a cyano group, a non-radioactive halogen substituent, an alkyl substituent having 1 to 4 carbon atoms, and an alkoxy substituent having 1 to 4 carbon atoms; and R2 represents a radioactive halogen substituent, provided that at least one of A1, A2, A3 and A4 is a carbon and then R1 binds to the at least one of A1, A2, A3 and A4 which is a carbon.