The present invention provides a compound which has affinity with amyloid, shows sufficiently rapid clearance from normal tissues and is suppressed in toxicity such as mutagencity. Provided is a compound represented by the following formula (1) or a salt thereof: wherein A1, A2, A3 and A4 independently represents a carbon or nitrogen; R1 is a halogen substituent; R2 is a halogen substituent; and m is an integer of 0 to 2, provided that at least one of R1 and R2 is a radioactive halogen substituent, 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 as well as a low-toxic diagnostic agent comprising a compound represented by the preceding formula or a salt thereof.