Abstract: A compound that has affinity with amyloid, exhibits sufficiently rapid clearance from normal tissues, and is suppressed in toxicity such as mutagenicity is provided, which is represented by the following formula (1): or a salt thereof, wherein R1 is a group selected from hydrogen, hydroxyl group, carboxyl group, sulfate group, amino group, nitro group, cyano group, an alkyl substituent with one to 4 carbon atoms or an alkoxy substituent with one to 4 carbon atoms; R2 is a radioactive halogen substituent; and m is an integer of 0 to 2, and a low-toxic diagnostic agent for Alzheimer's disease comprising a compound represented by the above formula or a salt thereof is also provided.