The present invention pertains to stabilization of a certain class of benzothia(dia)zepine derivative in a solid preparation containing the derivative. The present invention provides a solid preparation containing (A) the benzothia(dia)zepine derivative, and (B) a specific plasticizer, wherein either the component (A) and the component (B) are isolated, or, when the component (A) and the component (B) are not isolated, the component (B) content is no more than 0.9 wt.% based on the total weight of the solid preparation. The solid preparation is formed so as to be provided with at least one core, and at least one sheath layer or capsule layer enveloping at least a portion of the core, the component (A) being blended into the core, and the component (B) being blended into the sheath layer or capsule layer. The component (B) content of the sheath layer or capsule layer is no more than 45 wt.% based on the total weight of the sheath layer or capsule layer.
