Provided is a stable pharmaceutical composition for oral administration comprising 6-(4,4-dimethylcyclohexyl)-4-[(1,1-dioxo-1λ6-thiomorpholin-4-yl)methyl]-2-methylthieno[2,3-d]pyrimidine or a pharmaceutically acceptable salt thereof, having rapid drug dissolution properties. The pharmaceutical composition for oral administration comprises a water-swellable substance, which is a polymer compound obtained by condensation polymerization of β-glucose, a polymer compound obtained by condensation polymerization of α-glucose, or a polymer compound having a pyrrolidone functional group.本發明係提供在含有6-(4,4-二甲基環己基)-4-[(1,1-二側氧基-1λ6-硫代嗎啉-4-基)甲基]-2-甲基噻吩并[2,3-d]嘧啶或其製藥學上可容許的鹽之經口投予用醫藥組成物中，顯示出迅速的藥物溶出性之安定的經口投予用醫藥組成物。前述經口投予用醫藥組成物含有水膨潤性物質，前述水膨潤性物質為β-葡萄糖縮聚而得之高分子化合物、α-葡萄糖縮聚而得之高分子化合物或具有吡咯啶酮官能基之高分子化合物。
