The invention relates to a method for forming a transparent conductive thin film of In 2 O 3 -SnO 2 on a surface of a plastics substrate of less heat resistance other than that of glass, ceramics, etc. When an In 2 O 3 -SnO 2 precursor sol is produced by hydrolyzing and polymerizing a solution containing indium alkoxide and tin alkoxide, either tri-s-butoxyindium or tri-t-butoxyindium is used as the indium alkoxide. water is added to the solution containing indium alkoxide and tin alkoxide at a temperature of not higher than -20°C. The obtained In 2 O 3 -SnO 2 precursor sol is applied to a surface of a substrate to form a gel film, then the gel film is either irradiated with an ultraviolet beam of which wave length is not longer than 360 nm, or irradiated with an ultraviolet beam of which wave length is not longer than 260 nm and further irradiated with a laser beam of which wave length is not longer than 360 nm, to crystallize the gel forming the thin film, whereby an In 2 O 3 -SnO 2 thin film having a conductivity is formed on the surface of the substrate.