To obtain a composition which can form at good efficiency a cured coating film having excellent adhesion to substrates, scratch resistance and weatherability and a low refractive index by using a mercaptosilane-modified fluoro copolymer obtained by reacting an epoxy fluoro copolymer with a silane compound.

This composition comprises a mercaptosilane-modified fluoro copolymer obtained by reacting 100 pts.wt. epoxy fluoro copolymer obtained by polymerizing a mixture containing 20-70mol% hexafluoropropylene, 1-20mol% epoxy monomer and 10-70mol% monomers copolymerizable therewith and having a fluorine content of 40wt.% or above and an intrinsic viscosity of 0.05-2dl/g (as measured in N,N-dimethylacetamide at 25°C) with 5-50 pts.wt. hydrolyzable silane compound having at least one mercapto group in the molecule.