Disclosed is a compound represented by the formula (1) or a salt thereof, which has an affinity to amyloid, shows a satisfactorily rapid clearance from a normal tissue and is reduced in toxicity such as mutagenicity. Also disclosed is a low-toxic diagnostic agent for Alzheimer's disease comprising the compound. (1) [wherein A, A, A and A independently represent a carbon or a nitrogen; and R represents a group represented by the formula: (wherein R represents a radioactive halogen substituent; m represents an integer of 0 to 4; and n represents an integer of 0 or 1) provided that at least one of A, A, A and A represents a carbon and R is bound to the carbon A, A, A or A].