It is intended to provide a method for significantly improving the bioavailability of orally administered sapropterin hydrochloride which is used as a therapeutic agent for BH4 responsive hyperphenylalaninemia and a preparation for oral administration or a preparation for oral intake with improved bioavailability. It is a preparation in which the bioavailability of sapropterin hydrochloride has been improved characterized by containing sapropterin hydrochloride and an absorption promoter comprising an organic carboxylic acid having a plurality of carboxyl groups in its molecule, wherein the organic carboxylic acid containing a plurality of carboxyl groups in its molecule is selected from tartaric acid, citric acid, malic acid, or an optically active substance, a racemic compound, an anhydride or a hydrate thereof or a mixture thereof and the addition amount thereof is 0.1 to 100 times the weight of sapropterin hydrochloride.