Disclosed are 2-phenyl-imidazo[1,2-a]pyridine derivatives as represented by the following formula (1), or a salt thereof: wherein: all of A1, A2, A3 and A4 represent carbons, R1 is a radioactive halogen substituent R2 is a group selected from the group consisting of hydrogen, cyano, hydroxyl, methoxy, or is a carboxyl, amino, N-methylamino or N,N-dimethylamino group p is an integer of 0 to 2. Further disclosed are representative compounds of formula (1) such as 2-(4-ethoxyphenyl)-6-[123I]iodoimidazo[1,2-a]pyridine, 2-[4-(2-hydroxyethoxy)phenyl]-6-[125I]iodoimidazo[1,2-a]pyridine, 2-[4-(3-hydroxypropoxy)phenyl]-6-[131I]iodoimidazo[1,2-a]pyridine. Further disclosed is a diagnostic agent for Alzheimers disease, which comprises a compound as defined above, or a salt thereof.
