The present invention is: 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 sulfur atom in its molecule. This invention provides: a curable composition that produces a cured product that exhibits excellent adhesion, excellent delamination resistance, and excellent heat resistance, and exhibits excellent workability during the application step, a method for producing the same, a cured product obtained by curing the curable composition, a method for using the curable composition as an optical element-securing adhesive or an optical element sealing material, and an optical device.