Morphinan derivative represented by the following general formula (I): ** ** Formula (wherein, in the formula, R1 represents hydrogen, C1-10alkyl, C6-10aryl, C2-6 alkenyl, cycloalkylalkyl (the rest cycloalkyl having 3 to 6 carbon atoms, and alkylene radical having 1 to 5 carbon atoms), aralkyl (the aryl moiety having 6 to 10 carbon atoms, and alkylene radical having 1 to 5 carbon atoms ) alkyl, C3- 6, or heteroaryl (heteroaryl containing 1 to 4 heteroatoms selected from N, O and S atoms as ring constituents, and the alkylene radical having 1 to 5 carbon atoms), R2 represents hydrogen, C1-10, C3-6 cycloalkyl, C6-10 aryl, heteroaryl (containing 1 to 4 heteroatoms selected from N, O and S as ring-constituting atoms), aralkyl (the aryl moiety having 6 to 10 carbon atoms , and the alkylene radical having 1 to 5 carbon atoms), heteroaryl (heteroaryl cont iene 1 to 4 heteroatoms selected from N, O and S as constituents of the ring atoms, and alkylene radical having 1 to 5 carbon atoms), cycloalkylalkyl (the cycloalkyl moiety has 3 to 6 carbon atoms, and the rest alkylene having 1 to 5 carbon atoms), C2-6 alkenyl, arylalkenyl (the aryl moiety having 6 to 10 carbon atoms, and alkenyl radical having 2 to 6 carbon atoms), heteroaryl (the heteroaryl contains 1 to 4 heteroatoms selected from N, O and S as constituents of the ring atoms, and alkenyl radical having 2 to 6 carbon atoms), cycloalkylalkenyl (the cycloalkyl moiety has 3 to 6 carbon atoms, and alkenyl moiety has from 2 to 6 carbon atoms), C4-6 cycloalkenyl, cycloalkenylalkyl (the cycloalkenyl moiety has 4 to 6 carbon atoms, and alkylene radical having 1 to 5 carbon atoms), or cycloalkenylalkenyl (the cycloalkenyl moiety has 4 to 6 carbon atoms, and alkenyl radical having 2 to 6 carbon atoms), R3, R4 and R5, which are identical or different, represent hydrogen, hydroxy, halogen, cyano, carbamoyl, C1-6 alkoxy, C6-10 aryloxy, nitro, amino or acylamino (acyl moiety has from 2 to 6 carbon atoms), R6a and R6b represent hydro
