A morphinan derivative represented by general formula (I) or an acid addition salt thereof is used as an analgesic. (In the formula, R1 represents a hydrogen atom, a C1-6 alkyl group, a cycloalkyl alkyl group (wherein the cycloalkyl moiety has 3-6 carbon atoms and the alkylene moiety has 1-5 carbon atoms), an aralkyl group (wherein the aryl moiety has 6-10 carbon atoms and the alkylene moiety has 1-5 carbon atoms) or the like R2 represents a hydrogen atom, a C1-6 alkyl group, a C3-6 cycloalkyl group, a C6-10 aryl group, an aralkyl group (wherein the aryl moiety has 6-10 carbon atoms and the alkylene moiety has 1-5 carbon atoms) or the like each of R3, R4 and R5 represents a hydrogen atom, a hydroxy group, a carbamoyl group, a C1-6 alkoxy group, a C6-10 aryloxy group or the like each of R6a, R6b, R7, R8, R9 and R10 represents a hydrogen atom or the like X represents O or CH2 and Y represents C=O, SO2, a bonding hand or the like.)
