To provide a method for producing a thin gel coating film which has a smooth surface and excellent mechanical physical properties such that it can contain a specific medium and solute.

The organic-inorganic composite gel in a slurry state is prepared by polymerizing a water-soluble radical polymerizable monomer under the coexistence of a water-swellable clay mineral with a predetermined amount of water to synthesize an organic-inorganic composite gel; and pulverizing the organic-inorganic composite gel after swelling the organic-inorganic composite gel so as to have a water medium amount in a specific range, or after further adding water medium after the swelling. An organic-inorganic composite gel coating film containing the intended medium and solute is obtained by forming a coating film by use of this slurry organic-inorganic composite gel, and impregnating it with the medium or the medium containing the solute.