An oral preparation which is a dosage form selected from the group consisting of tablets, capsules, dry syrups, granules and powders, comprising an organic acid and as an effective ingredient a morphinan derivative having a nitrogen-containing heterocyclic group represented by the Formula: [wherein R 1 is hydrogen, C 1 -C 5 alkyl, C 4 -C 7 cycloalkylalkyl, C 6 -C 8 cycloalkenylalkyl, C 6 -C 12 aryl, C 7 -C 13 aralkyl, C 3 -C 7 alkenyl, furanylalkyl (wherein the number of carbon atoms in the alkyl moiety is 1 to 5), thienylalkyl (wherein the number of carbon atoms in the alkyl moiety is 1 to 5) or pyridylalkyl (wherein the number of carbon atoms in the alkyl moiety is 1 to 5); R 2 and R 3 independently are hydrogen, hydroxy, C 1 -C 5 alkoxy, C 3 -C 7 alkenyloxy, C 7 -C 13 aralkyloxy or C 1 -C 5 alkanoyloxy; Y and Z independently represent valence bond or -C(=O)-; -X- represents a C 2 -C 7 carbon chain (one or more of the carbon atoms therein optionally is(are) replaced by nitrogen, oxygen or sulfur atom(s), and the carbon chain optionally contains an unsaturated bond) constituting a part of the ring structure; k is an integer of 0 to 8; R 4 is(are) (a) substituent(s) in the number of k on the nitrogen-containing ring, which independently represent(s) fluorine, chlorine, bromine, iodine, nitro, hydroxy, C 1 -C 5 alkyl, C 7 -C 13 cycloalkylalkyl, C 6 -C 12 aryl, C 7 -C 13 aralkyl, C 7 -C 13 aralkyloxy, C 1 -C 5 alkoxy, trifluoromethyl, trifluoromethoxy, cyano, isothiocyanato, SR 6, SOR 6, SO 2 R 6, (CH 2) p OR 6, (CH 2) p COR 6, (CH 2) p CO 2 R 6, SO 2 NR 7 R 8, CONR 7 R 8, (CH 2) p NR 7 R 8 or (CH 2) p N(R 7)COR 8, or among the R 4 s in the number of k, two R 4 s bound to the same carbon atom or to the same sulfur atom together represent one oxygen atom to form carbonyl or sulfoxide (with the proviso that in cases where Y or Z is valence bond, the formed carbonyl is not bound directly to the nitrogen atom which is bound to the morphinan structure), or two R 4 s bound to the same carbon atom together represent one sulfur atom to form thiocarbonyl, or four R 4 s bound to the same sulfur atom together represent two oxygen atoms to form sulfone, or among the R 4 s in the number of k, two R 4 s bound to adjacent carbon atoms, respectively, together form benzene fused ring, pyridine fused ring, naphthalene fused ring, cyclopropane fused ring, cyclobutane fused ring, cyclopentane fused ring, cyclopentene fused ring, cyclohexane fused ring, cyclohexene fused ring, cycloheptane fused ring or cycloheptene fused ring, each of said fused rings is non-substituted or substituted by 1 or more R 5 s, wherein R 5 (s) independently represent(s) fluorine, chlorine, bromine, iodine, nitro, hydroxy, C 1 -C 5 alkyl, C 1 -C 5 alkoxy, trifluoromethyl, trifluoromethoxy, cyano, C 6 -C 12 aryl, C 7 -C 13 aralkyl, isothiocyanato, SR 6, SOR 6, SO 2 R 6, (CH 2) p OR 6, (CH 2) p COR 6, (CH 2) p CO 2 R 6, SO 2 NR 7 R 8, CONR 7 R 8, (CH 2) p NR 7 R 8 or (CH 2) p N(R 7)COR 8; R 9 is hydrogen, C 1 -C 5 alkyl, C 1 -C 5 alkenyl, C 7 -C 13 aralkyl, C 1 -C 3 hydroxyalkyl, (CH 2) p OR 6 or (CH 2) p CO 2 R 6; R 10 and R 11 are bound to form -O-, -S- or - CH 2 -, or R 10 is hydrogen and R 11 is hydrogen, hydroxy, C 1 -C 5 alkoxy or C 1 -C 5 alkanoyloxy; p is an integer of 0 to 5; R 6 is hydrogen, C 1 -C 5 alkyl, C 3 -C 7 alkenyl, C 6 -C 12 aryl or C 7 -C 13 aralkyl; and R 7 and R 8 independently are hydrogen, C 1 -C 5 alkyl or C 7 -C 13 aralkyl] or a pharmaceutically acceptable acid addition salt thereof, wherein 1g of said organic acid requires not less than 30 mL of water to dissolve in at 20°C; wherein said organic acid is one selected from the group consisting of fumaric acid, L-glutamic acid, L-aspartic acid and benzoic acid, or a mixture of two or more of these; and wherein the content of said organic acid is 0.01 to 60% by weight based on the total weight of the oral preparation. wherein said organic acid is one selected from the group consisting of fumaric acid, L-glutamic acid, L-aspartic acid and benzoic acid, or a mixture of two or more of these; and wherein the content of said organic acid is 0.01 to 60% by weight based on the total weight of the oral preparation.