Abstract: The invention relates to a sol-gel method for forming a metal oxide film, e.g. TiO.sub.2 film or TiO.sub.2 -SiO.sub.2 film, on a substrate by applying a solution of at least one metal alkoxide to the substrate to form a sol film on the substrate, drying the sol film to cause it to turn into a gel film by hydrolysis and baking the gel film. According to the invention the viscosity of the metal alkoxide solution is adequately increased, preferably to 3-30 cP at 20.degree. C., by the addition of a combination of a hydroxypropyl cellulose of which 2 wt % aqueous solution has a viscosity of 150-400 cP at 20.degree. C. and another hydroxypropyl cellulose of which 2 wt % aqueous solution has a viscosity of 6-10 cP at 20.degree. C. Metal oxide films formed by this method are free from minute cracks and excellent in optical characteristics and durability. By this method it is possible to form a metal oxide film as thick as about 300 nm without repeating the application of the alkoxide solution to the substrate.