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 R10, which are the same or different, represent hydrogen, etc.,X represents O or CH2, andY 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.