It is intended to provide a reagent for detecting amyloids in a biological tissue by which amyloids can be detected in vitro and in vivo at a high sensitivity using a compound having amyloid affinity and having been regulated in toxicity such as mutagenicity. A reagent for detecting amyloids deposited in a biological tissue which comprises a compound represented by the following general formula (1) or its salt. (1) wherein A1, A2, A3 and A4 each independently represents carbon or nitrogen; R1 represents a radioactive halogen substituent; R2 represents a group selected from the group consisting of hydrogen, hydroxyl, methoxy, carboxyl, amino, N-methylamino, N,N-dimethylamino and cyano; and m is an integer of 0 to 2; provided that at least one of A1, A2, A3 and A4 is hydrogen and R1 is bonded to A1, A2, A3 or A4 that is carbon.