The invention relates to a reflectance reducing silicon dioxide film formed on a glass substrate by the sol-gel process. To make the film minutely rough, at least two sols are respectively prepared from at least one compound selected from the group consisting of silicon alkoxides and silicon 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 a sol film on the glass substrate. The thus coated glass substrate is heated so as to transform the sol film into a gel film. The gel film which is minutely rough is satisfactory in reflectance reduction.