A solution is prepared that contains (A) a polymer compound that includes at least one of a hydrolyzable polymer compound or a thermally-decomposable polymer compound, (B) an oxoacid-based compound that includes at least one of a phosphate-based compound, a sulfate-based compound, a sulfonate-based compound, or a perchlorate-based compound, and (C) a laminate of layered substances, and the solution is irradiated with at least one of sonic waves or radio waves, or the solution is heated.