The coating solution of the invention is useful for forming a silica-based coating layer on a substrate such as semiconductor silicon wafers in the manufacturing process of semiconductor devices such as VLSIs. The coating solution is particularly advantageous to smooth a substrate surface having a difference in levels by completely filling the recessed areas. The coating solution is an organic solution of a cohydrolyzate of an alkoxy silane mixture composed of at least two kinds of di-, tri- and tetraalkoxy silane compounds such as a combination of methyl trimethoxy silane and tetramethoxy silane in a specified molar ratio and can be prepared by adding water to an organic solution of these alkoxy silane compounds without using any acid catalyst to effect the cohydrolysis of the silane compounds.