The present invention relates to stabilization of a certain benzothia(dia)zepine derivative in a solid formulation containing the same. In the present invention, in a solid formulation containing (A) the benzothia(dia) zepine derivative mentioned above and (B) a specific plasticizer, the aforementioned ingredient (A) is isolated from the aforementioned ingredient (B), or alternatively, in the case of the aforementioned ingredient (A) not being isolated from the aforementioned ingredient (B), an amount of the aforementioned ingredient (B) is controlled to 0.9% by weight or less based on the total weight of the solid formulation, or the solid formulation is provided with a form containing at least one core and at least one coating layer or capsule layer enclosing at least a part of the core, the aforementioned ingredient (A) is mixed into the core, the aforementioned ingredient (B) is mixed into the coating layer or capsule layer, and an amount of the aforementioned ingredient (B) in the coating layer or capsule layer is controlled to 45% by weight or less based on the total weight of the coating layer or capsule layer.
