A morphinan derivative represented by the following general formula (I): (wherein R 1 represents hydrogen, C 1-10 alkyl, cycloalkylalkyl where the cycloalkyl moiety has 3 to 6 carbon atoms, and the alkylene moiety has 1 to 5 carbon atoms, etc., R 2 represents heterocyclic ring containing 1 to 4 heteroatoms selected from N, O and S and at least one carbon atom as ring-constituting atoms, containing at least one set of adjacent ring-constituting atoms bound by a double bond, and further substituted with at least one oxo group, Y binds to a carbon atom as a ring-constituting atom of R 2 , R 3 , R 4 , and R 5 represent hydrogen； hydroxy, etc., R 6a and R 6b represent hydrogen, etc., R 7 and R 8 represent hydrogen, etc., R 9 and R 10 , which are the same or different, represent hydrogen, etc., X represents O or CH 2 , and Y represents C(=O)), a tautomer or stereoisomer of the compound, or a pharmaceutically acceptable salt thereof, or a solvate thereof is used as an anxiolytic drug, antidepressant, etc.
