A chemically stable oral preparation with an excellent dissolution property comprising as an effective ingredient a specific morphinan derivative or a pharmaceutically acceptable acid addition salt thereof is disclosed. The oral preparation according to the present invention comprises a specific morphinan derivative or a pharmaceutically acceptable acid addition salt thereof as an effective ingredient and an organic acid, wherein 1 g of said organic acid requires not less than 30 mL of water to dissolve in at 20°C. The method for improving dissolution property and chemical stability of an oral preparation according to the present invention comprises incorporating an organic acid in the oral preparation comprising as an effective ingredient a specific morphinan derivative or a pharmaceutically acceptable acid addition salt thereof, wherein 1 g of said organic acid requires not less than 30 mL of water to dissolve in at 20°C.
