Use of a morphinan derivative of Formula (I) having a nitrogen-containing heterocyclic group: ** Formula ** [in which R 1 is hydrogen, C 1 -C 5 alkyl, C 4 -C 7 cycloalkylalkyl, C 6 -C 8 cycloalkenyl alkyl, aryl C6-C12, aralkyl C7-C13, alkenyl C3-C7, furanylalkyl (in which the number of carbon atoms in the alkyl moiety is 1 to 5), thienyl alkyl (in which the number of carbon atoms in the rest alkyl is from 1 to 5) or pyridylalkyl (in which the number of carbon atoms in the alkyl moiety is from 1 to 5), R2 and R3 independently are hydrogen, hydroxy, C1-C5 alkoxy, C3-C7 alkenyloxy, aralkyloxy C7-C13 or C1-C5 alkanoyloxy； Y and Z independently represent a valence bond or -C (>； = O) -； -X- represents a C2-C7 carbon chain (one or more of the carbon atoms therein may be replaced by an atom or atoms of nitrogen, oxygen or sulfur, and the carbon chain may contain an unsaturated bond) that constitutes a part of the annular structure； k is an integer from 0 to 8； R4 is or is (a) a substituent or substituents in the number of k in the nitrogen-containing ring, which independently represents or represents fluorine, chlorine, bromine, iodine, nitro, hydroxy, C1-C5 alkyl, C7-C13 cycloalkylalkyl, C6-C12 aryl, C7-C13 aralkyl, C7-C13 aralkyloxy, C1-C5 alkoxy, trifluoromethyl, trifluoromethoxy, cyano, isothiocyanate, SR6, SOR6, SO2R6, (CH2) pOR6, (CH2) pCOR6, (CH2) SO2 R2R2 , CONR7R8, (CH2) pNR7R8 or (CH2) pN (R7) COR8, or between R4 in the number of k, two R4 attached to the same carbon atom or the same sulfur atom together represent an oxygen atom to form carbonyl or sulfoxide (with the proviso that in cases where Y or Z is a valence bond, the carbonyl formed does not bind directly to the nitrogen atom that binds to the morphinan structure), or two R4 attached to it carbon atom together represent a sulfur atom to form thiocarbonyl, or four R4 attached to the same az atom ufre together represent two oxygen atoms to form sulfone, or between the R4 in the number of k, two R4 attache
