NOVELTY - A dispersant such as zinc compound, indium compound, gallium compound, stannic compound and/or thallic compound is dispersed in dispersing medium, to form dispersion. The obtained dispersion is stirred and aged. Thus, coating liquid is prepared. The molar ratio of zinc compound and indium compound is 1:0.1 and gallium compound and thallic compound is 1:2.    USE - Preparation of coating liquid used for forming oxide thin film such as indium-zinc oxide film, indium-gallium-zinc oxide film, indium-tin-zinc oxide film, tin-zinc oxide film, tin-gallium-zinc oxide film, thallium-gallium-zinc oxide film and thallium-zinc oxide film used for organic light-emitting diode, liquid crystal display and solar cell.    ADVANTAGE - The method simply and economically provides coating liquid in high yield by sol gel method.    DETAILED DESCRIPTION - A dispersant such as zinc compound, indium compound, gallium compound, stannic compound and/or thallic compound is dispersed in dispersing medium, to form dispersion. The obtained dispersion is stirred and aged. Thus, coating liquid is prepared. The molar ratio of zinc compound and indium compound is 1:0.1 and gallium compound and thallic compound is 1:2. The dispersing medium is isopropanol, 2- methoxyethanol, dimethylformamide, ethanol, deionized water, methanol, acetyl acetone, dimethylamine borane and/or acetonitrile. The zinc compound is zinc citrate dihydrate, zinc acetate, zinc acetate dihydrate, zinc acetylacetonate hydrate, zinc acrylate, zinc chloride, zinc diethyldithiocarbamate, zinc dimethyl dithiocarbamate, zinc fluoride, zinc fluoride hydrate, zinc hexafluoro acetylacetonate dihydrate, zinc methacrylate, zinc nitrate hexahydrate, zinc nitrate hydrate, zinc trifluoromethane sulfonate, zinc undecylenate, zinc trifluoroacetate hydrate, zinc tetrafluoroborate hydrate or zinc perchlorate hexahydrate. The indium compound is indium chloride, indium chloride tetrahydrate, indium fluoride, indium fluoride trihydrate, indium hydroxide, indium nitrate hydrate, indium acetate hydrate, indium acetylacetonate or indium acetate. The gallium compound is gallium acetylacetonate, gallium chloride, gallium fluoride or gallium nitrate hydrate. The stannic compound is tin acetate, tin chloride, tin chloride dihydrate, tin chloride pentahydrate or tin fluoride. The thallium compound is thallium acetate, thallium acetylacetonate, thallium chloride, thallium chloride tetrahydrate, thallium cyclopentadienide, thallium fluoride, thallium formate, thallium hexa fluoro acetylacetonate, thallium nitrate, thallium nitrate trihydrate, thallium trifluoroacetate or thallium perchlorate hydrate. The sol stabilizer such as monoethanolamine and diethanolamine or triethanolamine is mixed with dispersion. The pH of dispersion is reduced by adding acetic acid and increased by adding ammonium hydroxide, potassium hydroxide or sodium hydroxide.