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.