The invention provides a reagent which can detect amyloid in vitro and in vivo with high sensitivity using a compound which has affinity with amyloid.The reagent for detecting amyloid deposited in a biological tissue comprises the compound represented by the following formula (1) or a salt thereof:wherein A1, A2, A3 and A4 independently represent a carbon or nitrogen,R1 is a group selected from the group consisting of a hydrogen, hydroxyl group, carboxyl group, sulfuric acid group, amino group, nitro group, cyano group, non-radioactive halogen substituent, alkyl substituent with 1 to 4 carbon atoms and alkoxy substituent, with 1 to 4 carbon atoms, andR2 is a radioactive halogen substituent,provided that at least one of A1, A2, A3 and A4 represents a carbon, and R1 binds to a carbon represented by A1, A2, A3 or A4.
