The present invention is a sustained release formulation which is capable of controlling dissolution of a drug and of enduring peristaltic movement of the gastrointestinal tract. Specifically, the present invention is: a solid dosage form comprising a drug, croscarmellose sodium, an alkali metal salt or an alkoxide, and an alkali agent, wherein the alkali metal salt is a sodium salt or a potassium salt, and the alkali agent is a magnesium oxide； or a solid dosage form comprising a drug, croscarmellose sodium, an alkali metal salt or an alkoxide, and a water soluble polymer or a hydrophobic base, wherein the alkali metal salt is a sodium salt or a potassium salt.
