There is provided a stable sol of zirconium oxide-tin oxide composite colloidal particles and a method of producing the same, a hard coat material applied for a surface of a plastic lens, and an optical member.The present invention relates to a sol comprising zirconium oxide-tin oxide composite colloidal particles (A), wherein a molar ratio of oxide in the zirconium oxide-tin oxide composite colloidal particles (A) is 0.02-0.4 as SnO2/ZrO2, and the zirconium oxide-tin oxide composite colloidal particles (A) have a structure in which zirconium oxide colloidal particles and tin oxide colloidal particles are bonded together, and a primary particle diameter of 2-100 nm; and also relays to a coating composition comprising a silicon-containing substance (S) of an organic silicon compound or a hydrolysate thereof, and a sol comprising zirconium oxide-tin oxide composite colloidal particles (A), and an optical member having a cured coating formed from the coating composition.