L-threo-.beta.-benzyloxyaspartate derivatives having a substituent on the benzene ring, represented by the following formula (1): ##STR00001## wherein R is hydrogen, a linear or branched lower aliphatic acyl group with the acyl portion optionally substituted, an alicyclic acyl group, an aromatic acyl group with a substituent on the aromatic ring, an amino acid-derived group or a biotin derivative-derived group, having an amino substituent on the benzene ring, and salts thereof, which can easily bind to affinity column chromatography carriers as ligands of glutamate transporter proteins.