To provide a cationic curable composition which is excellent in curability, has a readily controllable thermal expansion coefficient, exhibits excellent heat resistance and scratch resistance and forms a cured product such as a film or the like.

The cationic curable composition comprises a polyorganosiloxane bearing an oxetanyl group and substantially no hydrolyzable group, a cationically polymerizable monomer, a cationic polymerization initiator and an inorganic filler (silica or the like). Preferably, the polyorganosiloxane bearing the oxetanyl group is a hydrolyzate condensate of an organic silicon-containing compound represented by general formula (1) or a hydrolyzate condensate of the organic silicon-containing compound with an organic silicon-containing compound bearing a siloxane bond-forming group and no oxetanyl group.