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