Disclosed is a chemically stable oral preparation having an excellent elutionproperty, which comprises a specific morphinan derivative or a pharmacologicallyacceptable acid addition salt thereof as an active ingredient. Specificallydisclosed is an oral preparation which comprises a specific morphinan derivativeor a pharmacologically acceptable acid addition salt thereof as an active ingredient,and additionally contains such an organic acid that 30 mL or more of water is neededfor dissolving 1 g of the organic acid at 20˚C. Also disclosed is a methodfor improving the elution property and the chemical stability of an oral preparationcomprising a specific morphinan derivative or a pharmacologically acceptableacid addition salt thereof as an active ingredient, which comprises adding suchan organic acid that 30 mL or more of water is needed for dissolving 1 g of the organicacid at 20˚C to the oral preparation.
