A curable composition comprising a component (A), a component (B), and a component (C), the curable composition comprising the component (A) and the component (B) in a mass ratio (component (A):component (B)) of 100:0.3 to 100:50,
- the component (A) being a curable polysilsesquioxane compound that comprises a repeating unit represented by a formula (a-1),
- R1SiO3/2 (a-1)
- wherein R1 is a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, or a substituted or unsubstituted aryl group,
- the component (B) being fine particles having an average primary particle size of 5 to 40 nm, and
- the component (C) being a silane coupling agent that comprises a nitrogen atom in its molecule.