A sol-form application liquid 3 is prepared by mixing together two solvents A and B and two film components C and D. The sol-form application liquid 3 thus prepared is applied onto a glass substrate 200, thus forming a mixed layer 401 on the glass substrate 200. The glass substrate 200 having the mixed layer 401 formed thereon is left at room temperature, whereupon the solvent A evaporates and phase separation of the mixed layer 401 into an upper layer 403 and a lower layer 402 occurs. The mixed layer 401 is then heated, thus evaporating the solvent B in the lower layer 402 and gelating the film component C in the upper layer 403 and the film component D in the lower layer 402, and hence forming an internal scattering layer 404.