An object of the present invention is to provide a film coating agent for asolid formulation, which has excellent water vapor barrier properties and iscapableof stably maintaining the quality of medicinal ingredients even when the solidformulation has been stored unpacked for a long period of time. The presentinvention provides a film coating agent for a solid formulation, comprising awater-soluble cellulose derivative, swelling clay, a cationic surfactant, anda fatty acid,wherein the mass ratio of the above-described swelling clay to the above-describedwater-soluble cellulose derivative is 2:8 to 8:2, and the content of theabove-described cationic surfactant is not less than 0.5 equivalents and notmore than 3.0equivalents relative to the cation exchange equivalent of the above-describedswelling clay.
