TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Agent: The curable composition further contains radical generating agent which generates radical by light irradiation. Preferred Component: The cage form silsesquioxane structure is preferably of formula: (RSiO1.5)c, and has content of less than 30 mol%.c=6-20.Preferred Process: The curable composition is heated at 20-400 degrees C to form silsesquioxane cured material. Preferred Properties: The radical generating has 10-hour half-life temperature of 100-400 degrees C. The content of oligomer mixture is 1-60 mass parts with respect to sum total content of oligomer mixture, compounds (b) and (c). The molar ratio of silicon hydride group of compound (b) and sum of carbon-carbon double bond of compound (c) and oligomer mixture is 0.4-3. The molar ratio of carbon-carbon double bond of compound (c) and carbon-carbon double bond of oligomer mixture is 2-200. The oligomer mixture has number average molecular weight of 500-4000, and molecular weight distribution of 1-1.5. The cured material has total light transmittance of 85-100% at wavelength of 400-800 nm, maximum point elongation of 10-40% in tractive characteristics evaluation, and 5% weight reduction temperature of 400-800 degrees C. EXAMPLE - Tetrahydrofuran (in ml) (6000), sodium hydroxide (225), phenyl trimethoxysilane (333.13 g) and vinyl trimethoxysilane (106.73 g) were heated at 63 degrees C for 18 hours, 1N hydrochloric acid was added, filtered and dried to obtain cage form silsesquioxane oligomer having number average molecular weight of 1100 and molecular weight distribution of 1.2. The obtained oligomer (14.99 g), divinyl benzene (2.5 g) chloroplatinic acid ethanol solution (20 mu l) and 1,3,5,7-tetramethyl cyclotetrasiloxane (4.13) were degassed to form curable composition. The obtained composition was heated at 120 degrees C for 30 minutes and 200 degrees C for 2 hours to obtain cured material having total light transmittance of 92%, maximum point elongation of 8.3%, elastic modulus of 1.3 GPa, and 5% weight reduction temperature of 447 degrees C.