The present invention relates to the stabilization of a certain benzothia(dia)zepine derivative which is included in a solid formulation. According to the present invention, in a solid formulation including the (A) benzothia(dia)zepine derivative and (B) a specific plasticizer, the ingredients (A) and (B) are isolated from each other, or in the case of the ingredients (A) and (B) are not isolated from each other, the amount of the ingredient (B) is controlled to 0.9% by weight or less based on the total weight of the solid formulation, or providing the solid formulation with at least one core, and at least one coating layer or capsule layer enclosing the core, wherein the core comprises the ingredient (A) and the coating layer or capsule layer comprises the ingredient (B), and the amount of the 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.本發明係關於含有某種之苯并硫雜(二氮雜)呯衍生物的固體製劑中之該衍生物的安定化。本發明係於含有(A)前述苯并硫雜(二氮雜)呯衍生物、以及(B)特定之可塑劑的固體製劑中，將前述(A)成分及前述(B)成分隔離，或未將前述(A)成分及前述(B)成分隔離的情況時，將前述(B)成分之含量，以固體製劑之總重量為基準，設為0.9重量%以下；或者使前述固體製劑成為具備至少1個核心、及包圍前述核心之至少一部分的至少1個被覆層或膠囊層之形態，於前述核心中摻合前述(A)成分，於前述被覆層或膠囊層中摻合前述(B)成分，且將前述被覆層或膠囊層中之前述(B)成分之含量，以前述被覆層或膠囊層之總重量為基準，設為45重量%以下。
