A coating composition comprising inorganic oxide fine particles containing at least one element selected from the group consisting of Ti, Zr, Sn and Sb, a hydrolysable group-containing organic silicon compound, and (C1) a surfactant having an HLB value of 8 or less and (C2) a surfactant having an HLB value of more than 8, for forming a coat layer having little white turbidity, a good appearance and excellent scratch resistance, chemical resistance, hot water resistance and weather resistance on the surface of a plastic optical substrate.