The present invention provides a radiolabeled ligand which is highly selective and potent for glutamate transporters and is usable in specifically detecting the glutamate transporter.Specifically, the present invention provides a 3-[3-(benzoylamido)benzyloxy]aspartic acid having a radioactive substituent on the benzoyl group which is represented by the following formula (1), or an ester or salt thereof:wherein X represents a substituent containing a radioactive atom(s) which is selected from a straight or branched lower aliphatic alkyl group, a hydroxyl group, a straight or branched lower aliphatic alkoxy group, an amino group, a straight or branched lower aliphatic acylamido group, a halogen atom and a straight or branched lower aliphatic haloalkyl group and R1 and R2 each represents a hydrogen atom, a straight or branched lower aliphatic alkyl group or an acetoxymethyl group.
