The present invention provides a novel modified stannic oxide--zirconium oxide complex sol and a preparation method thereof, particularly a stable sol which contains 2-50% by weight of total metal oxides comprising modified colloidal particles of a stannic oxide--zirconium oxide complex having a particle diameter having 4.5-60 nm, wherein the particles are formed by coating complex colloidal particles of a stannic oxide--zirconium oxide having a bonded structure of stannic oxide colloidal particles and zirconium oxide colloidal particles at a ZrO.sub.2 /SnO.sub.2 ratio of 0.02-1.0 based on weights of these oxides and having a particle diameter of 4-50 nm as nuclei, with colloidal particles of a tungsten oxide--stannic oxide--silicon dioxide complex having such a constitution that a WO.sub.3 /SnO.sub.2 weight ratio is 0.1-100, that a SiO.sub.2 /SnO.sub.2 weight ratio is 0.1-100 and having a particle diameter of 2-7 nm, and preparation method thereof.