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 A 1 , A 2 , A 3 and A 4 independently represents a carbon or nitrogen; R<1> is a halogen substituent; R<2> is a halogen substituent; and m is an integer of 0 to 2, provided that at least one of R<1> and R<2> is a radioactive halogen substituent, at least one of A 1 , A 2 , A 3 and A 4 represents a carbon, and R<1> binds to a carbon represented by A 1 , A 2 , A 3 or A 4 as well as a low-toxic diagnostic agent comprising a compound represented by the preceding formula or a salt thereof.