The present invention aims to provide a coating agent which imparts gas barrier properties to a solid preparation while keeping rapid disintegration properties of the solid preparation, and a coating formed therewith, and to provide a coated solid preparation which maintains its rapid disintegration properties and gas barrier properties, and does not generate cracks in the coating under humidified conditions. The present invention provides a coating agent for a solid preparation, which coating agent contains lactose and a swelling clay, but does not contain a water-soluble polymer, wherein the weight of the swelling clay is 20 to 50% with respect to the weight of the solid content.
