The invention relates to a metal oxide film formed on a glass substrate by a sol-gel process. To make the metal oxide film minutely rough, at least two sols are respectively prepared from at least one compound selected from the group consisting of metal alkoxides and metal acetylacetonates, such that polymers of the sols have different average molecular weights. Then, the at least two sols are mixed with a solvent so as to prepare a coating solution. The coating solution is applied to the glass substrate so as to form thereon a sol film. The thus coated glass substrate is heated so as to transform the sol film into a gel film and to form thereon numerous micro-pits.