R.sub.f--SiX.sub.3 (D)[R.sub.f is a monovalent organic group containing one or more fluorine atoms; and X is a hydrolytic group].Inclusion of such a silane enables to further decrease a refractive index of a formed coating.In the general formula (D), the number of fluorine atoms in R.sub.f is 3 to 25, and preferably 3 to 17.