An object of the present invention is to provide a coating agent for a pharmaceutical solid preparation, which imparts an unpackaged pharmaceutical solid preparation with excellent barrier properties equivalent to those of a PTP sheet without affecting the disintegration properties of the pharmaceutical solid preparation. Provided is a coating agent for a pharmaceutical solid preparation which comprises a polyethylene glycol having an average molecular weight of 950 to 25,000 and a swelling clay, wherein the mass ratio of the polyethylene glycol and the swelling clay is 2:8 to 6:4.
