A morphinan derivative represented by the following general formula (I):wherein R1 represents hydrogen, C1-10 alkyl, cycloalkylalkyl where the cycloalkyl moiety has 3 to 6 carbon atoms, and the alkylene moiety has 1 to 5 carbon atoms, etc.； R2 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 R2； R3, R4, and R5 represent hydrogen, hydroxy, etc.； R6a and R6b represent hydrogen, etc.； R7 and R8 represent hydrogen, etc.； R9 and Rio, which are the same or different, represent hydrogen, etc.； X represents O or CH2； 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.
