The invention relates to a compound which has affinity with amyloid, shows sufficiently rapid clearance from normal tissues and is suppressed in toxicity such as mutagenicity, and also relates to a low-toxic diagnostic agent for Alzheimer's disease containing the compound. The compound is represented by the following formula (1) or a salt thereof: wherein A1, A2, A3 and A4 independently represent a carbon or a nitrogen, andR3 is a group represented by the following formula: wherein R1 is a radioactive halogen substituent; m is an integer of 0 to 4; and n is an integer of 0 or 1, provided that at least one of A1, A2, A3 and A4 represents a carbon, and R3 binds to a carbon represented by A1, A2, A3 or A4.