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 administrationA 2-amino-bicyclo[3.1.0]hexane-2,6-dicarboxylic ester derivative represented by formula [I] [wherein R1 and R2 are identical or different, and each represents a hydrogen atom, a C1-10alkyl group or the like; X represents a hydrogen atom or a fluorine atom; Y represents —OCHR3R4 or the like (wherein R3 and R4 are identical or different, and each represents a hydrogen atom, a C1-10alkyl group or the like; and n represents integer 1 or 2)], a pharmaceutically acceptable salt thereof or a hydrate thereof.