Abstract: A chitosan coating solution safe for oral administration, a water resistant chitosan coating film with reduced content of residual organic acid, and a coated medical preparation having the coating film are described. The preparation method includes incorporating a glycerin fatty acid ester into an aqueous chitosan solution prepared through dissolution with a volatile organic acid. The chitosan coating film is obtained by drying and solidifying the solution through film coating. The organic acid used in the dissolution remains in the coating film only minutely and the film is water resistant. When the chitosan coating solution (containing a glycerin fatty acid ester, and prepared through dissolution with a volatile organic acid) is coated, a film is formed therewith, and the organic acid volatilizes simultaneously, producing a chitosan water resistant coating film. Forming a known enteric coating film in combination with the outer coating film allows application to large-intestine drug delivery.