The morphinan derivatives represented by general formula (I) (wherein R1 represents hydrogen, a C1-10alkyl, a cycloalkylalkyl in which the cycloalkyl moiety has 3-6 carbon atoms and the alkylene moiety has 1-5 carbon atoms, or the like； R2 represents a heterocycle including at least one carbon atom and 1-4 heteroatoms selected from among N, O and S as constituent annular atoms, with at least one set of adjacent constituent annular atoms having a double bond, said heterocycle also including at least one oxo group as a substituent； Y is bonded to a carbon atom that is a constituent annular atom of R2； R3, R4 and R5 represent hydrogen, hydroxy or the like； R6a and R6b represent hydrogen or the like； R7 and R8 represent hydrogen or the like； R9 and R10 are the same or different, and represent hydrogen or the like； X represents O or CH2； and Y represents C(=O)), variants and stereoisomers of the derivatives, pharmaceutically acceptable salts thereof, and solvates thereof according to the present invention are used as antianxiety drugs, antidepressants and the like.
