The invention is a curable composition comprising the following component (A) and component (B):Component (A): a curable polysilsesquioxane compound having a repeating unit represented by the following specific formula (a-1): R1—D—SiO3/2  (a-1) Component (B): a silane coupling agent having a nitrogen atom in its molecule, anda method for producing the curable composition, anda cured product obtained by curing the curable composition, anda method for using the curable composition as an adhesive for an optical element-fixing material, anda method for using the curable composition as a sealant for an optical element-fixing material, andan optical device obtained by using the curable composition as an adhesive or a sealant for an optical element-fixing material.One aspect of the curable composition according to one embodiment of the invention can provide a cured product excellent in adhesiveness and heat resistance and having a low refractive index.