Coating agent as a film to a solid preparation and solid preparation using the same

Agent film coating for a solid formulation comprising a soluble cellulose derivative in water, swelling clay, a cationic surfactant and a fatty acid, wherein the mass ratio of said expansive clay with respect to said soluble cellulose derivative water is 2: 8 to 8: 2, and the content of said cationic surfactant is not less than 0.5 equivalents and not more than 1.5 equivalents relative to the cation exchange equivalent of said swelling clay in wherein said cationic surfactant is benzalkonium chloride, benzethonium chloride or distearyldimethylammonium chloride, and said fatty acid is a carboxylic acid compound monovalent linear long chain hydrocarbon.