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] [wherein R 1 and R 2 are identical or different, and each represents a hydrogen atom, a C 1-10 alkyl group or the like； X represents a hydrogen atom or a fluorine atom； Y represents -OCHR 3 R 4 or the like (wherein R 3 and R 4 are identical or different, and each represents a hydrogen atom, a C 1-10 alkyl group or the like； and n represents integer 1 or 2)], a pharmaceutically acceptable salt thereof or a hydrate thereof.
