A drug which is effective in treatments for and prevention of schizophrenia, anxiety, and diseases related to these, i.e., psychiatrical disorders such as depression, bipolar disorder, and epilepsy. The drug has high activity in oral administration and has an antagonistic effect on a Group II metabotropic glutamate receptor. It is a 2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic ester derivative represented by the formula [I]: [I] [wherein R1 and R2 are the same or different and each represents hydrogen, C1-10 alkyl, etc.; X represents hydrogen or fluorine; and Y represents, e.g., -OCHR3R4 (wherein R3 and R4 are the same or different and each represents hydrogen, C1-10 alkyl, etc. and n is an integer of 1 or 2)], a pharmaceutically acceptable salt of the derivative, or a hydrate of either.