There is provided a stable sol comprising modified stannic oxide-zirconium oxide complex colloidal particles, wherein the particles are formed by coating as nuclei stannic oxide-zirconium oxide complex colloidal particles in which stannic oxide colloidal particles obtained by a reaction of metal tin, an organic acid and hydrogen peroxide is bonded to zirconium oxide colloidal particles with tungsten oxide-stannic oxide-silicon dioxide complex colloidal particles. The sol has a high refractive index and is improved in water resistance, moisture resistance and weather resistance, and can be used by mixing with a hard coating paint as a component for improving properties of a hard coating film applied on a plastic lens surface.