A solid pharmaceutical preparation of solifenacin or a salt thereof, the preparation being stable and inhibited from decomposing with time when supplied to clinical fields. In a pharmaceutical preparation containing solifenacin or a salt thereof, the compound in an amorphous form was revealed to be causative of cardinal-drug decomposition with time. The composition for a solid pharmaceutical preparation of solifenacin or a salt thereof contains solifenacin or its salt each in a crystalline from, and the content provided are: a process for producing the composition； and a medicinal composition for solid pharmaceutical preparations which contains solifenacin and an amorphization inhibitor.
