A drug effective for the treatment and prevention of psychiatric disorders such as schizophrenia, anxiety and related ailments thereof, depression, bipolar disorder and epilepsy. The drug antagonizes the action of group II metabotropic glutamate receptors and shows high activity in oral administraiton A 2-amino-bicyclo [3.1.0] hexane-2,6-dicarboxylic ester derivative represented by formula [I] \n [wherein R 1 and R 2 are identical or different, and each represents a hydrogen atom, a C 1-10 alkyl group substituted by one or two aryl groups or other meanings as specified in the claims; X represents a fluorine atom; Y represents -OCHR 3 R 4 (wherein R 3 and R 4 are identical or different, and each represents a hydrogen atom or a phenyl group substituted by one to five halogen atoms, a pharmaceutically acceptable salt thereof or a hydrate thereof.
2-Amino-bicyclo[3.1.0]hexane-2,6-dicarboxylic ester derivative