An antidepressant comprising, as an active ingredient, a compound having an antagonistic effect on group II metabotropic glutamate receptors, as well as a 2-amino-3-alkoxy-6-fluoro-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivative of Formula [I]: [wherein R1 and R2, which may be the same or different, each represent a hydroxyl group, a C1-10 alkoxy group, etc.; R3 represents a C1-10 acyl group, a C1-6 alkoxy-C1-6 acyl group, etc.; and R4 and R5, which may be the same or different, each represent a hydrogen atom, a C1-10 alkyl group, etc.] or a pharmaceutically acceptable salt or hydrate thereof.