The present invention is a curable composition comprising a component (A) and a component (B), the curable composition producing a cured product that has a solid-state Si nuclear magnetic resonance spectrum in which a peak is observed within a range from −80 ppm or more to less than −40 ppm, and a half-width of the peak is 500 to 900 Hz, the component (A) being a curable polysilsesquioxane compound that comprises a repeating unit represented by a formula (a-1), R1SiO3/2  (a-1) the component (B) being at least one silane coupling agent selected from a group consisting of a silane coupling agent that comprises a nitrogen atom in its molecule, and a silane coupling agent that comprises an acid anhydride structure in its molecule, and a method for producing the curable composition, and a cured product, and a method for using the curable composition, and an optical device.