To provide a curable composition which is manufactured by a sol-gel method, comprises a polyorganosiloxane having an oxetane ring and a silsesquioxane structure, can glue a non-transparent base material such as a metal or a ceramic, can coat an article such as one having a curved surface, which cannot be uniformly irradiated with light, and can form a thick molded item, particularly a molded item having a thickness of at least several mm.

The thermosetting composition comprises (a), (b) and (c) as essential ingredients: (a) a polyorganosiloxane which is manufactured by the sol-gel method and has a functional group represented by formula (1) (wherein R0 is a hydrogen atom or a 1-6C alkyl group; and R1 is a 2-6C alkylene group) and a silsesquioxane structure; (b) an epoxy compound; and (c) a thermal cationic polymerization initiator.