PURPOSE: To obtain a thick and stable organosol containing particles of a tetravalent metal oxide, an amphiphilic acid system and a diluent, in which the amphiphilic acid system is an acid having a specified number of carbon atoms and containing branched chains at α, β, γ or δ positions to atoms each having acid hydrogen.

CONSTITUTION: The sol contains particles of a tetravalent metal oxide, an amphiphilic acid system and a diluent, in which the particles having a d90 of less than or equal to 200 nm and the sol has at least one of the following characteristics. (1) The particles of the tetravalent metal oxide are in the form of agglomerates of microcrystallites, advantageously in the form of agglomerates of CeO2 microcrystallites, and the d90 of the microcrystal, advantageously the d90 measured by photometric counting (high resolution transmission type electron microscopy) is less than or equal to 5 nm and 90 wt.% agglomerates contain 1-5 microcrystals. (2) The amphiphilic acid system contains at least one kind of acid being an acid having 11-50 carbon atoms and having at least one branch at α, β, γ or δ positions to atoms each having acid hydrogen.