The purpose of the present invention is to provide a coating agent and a film that is formed from the coating agent that impart gas barrier properties to a solid preparation while still ensuring fast disintegrability. The purpose of the present invention is also to provide a coated solid preparation wherein gas barrier properties and fast disintegrability are maintained and wherein cracks do not occur in a film under humidified conditions. The present invention provides a coating agent for a solid preparation, the coating agent including lactose and an expansive clay and not including a water-soluble polymer, the weight of the expansive clay being 20%-50% of the solid component weight.
