An oral preparation that is a dosage form selected from the group consisting of tablets, capsules, dry syrups, granules and powders, comprising a specific organic acid and, as an effective ingredient, a morphinan derivative having a heterocyclic group containing Nitrogen represented by Formula (I): ** Formula ** [wherein R1 is hydrogen, C1-C5 alkyl, C4-C7 cycloalkylalkyl, C6-C8 cycloalkenylalkyl, C6-C12 aryl, C3- alkenyl C7, furanylalkyl (where the number of carbon atoms in the alkyl moiety is 1 to 5), thienylalkyl (where the number of carbon atoms in the alkyl moiety is 1 to 5), or pyridylalkyl (where that 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, C7-C13 aralkyloxy 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 in it is (are) optionally (are) replaced by nitrogen, oxygen or sulfur atom (s) and the carbon chain optionally contains a bond unsaturated) constituting a part of the ring structure； k is an integer from 0 to 8； R4 is (are) (a / are) substituent / s in the number of k on the nitrogen-containing ring, which represents / n independently fluorine, chlorine, bromine, iodine, nitro, hydroxy, C1-C5-alkyl, C7-cycloalkylalkyl C13, C6-C12 aryl, C7-C13 aralkyl, C7-C13 aralkyloxy, C1-C5 alkoxy, trifluoromethyl, trifluoromethoxy, cyano, isothiocyanate, SR6, SOR6, SO2R6, (CH2) pOR6, (CH2) pCOR6, (CH2) p2 , SO2NR7R8, CONR7R8, (CH2) pNR7R8 20 or (CH2) pN (R7) COR8, or among the R4s in the number of k, two R4s attached to the same carbon atom or to the same sulfur atom together represent one atom of oxygen to form carbonyl or sulfoxide (provided that in cases where Y or Z is a valence bond, the carbonyl formed is not directly attached to the nitrogen atom that is attached to the morphinan structure), or two R4 attached to the same carbon atom together represent one
