TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The fluoro monomer (a) is perfluoro monomer, preferably tetrafluoroethylene. The fluorogen (b) is perfluoro diene or fluorine-containing compound of formula: CF2=CFO-Q1-OCF=CF2.Q1=3-8C perfluoro alkylene optionally having side chain of perfluoro alkyl or bivalent perfluoro alkylene optionally having ether oxygen atom between carbon atoms.Preferred Process: A fluorine-containing cured composition for optical medium and light-emitting element is obtained by hardening curable composition at 100-250 degrees C or by ultraviolet ray with wavelength of 150-400 nm. The fluorine-containing cured material is subjected to transparent sealing to form light-emitting element.    TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The fluoropolymer (X) has mass mean molecular weight of 3000-20000. EXAMPLE - Tetrafluoroethylene (in g) (21), perfluoro tetramethylene divinyl ether (78), AK225cb (RTM: 1,3-dichloro-1,1,2,2,3-pentafluoropropane) (1050), sulfur oxychloride (9) and perfluoro cyclohexane carbonyl peroxide (12) were mixed and polymerized at 50 degrees C. Then tetrafluoroethylene (51) and perfluoro tetramethylene divinyl ether (129) were added and polymerized under pressure of 0.13 MPa for 4 hours, to obtain polymerized mixture. The polymerized mixture was cooled, methanol (500) was added, stirred and copolymer was deposited. Then, supernatant liquid was added to AK225cb (RTM: 1,3-dichloro-1,1,2,2,3-pentafluoropropane), filtered and solvent was distilled, to obtain transparent highly viscous fluoropolymer (72). The obtained fluoropolymer was heated at 120 degrees C under vacuum for 2 hours, to obtain fluoropolymer (70) having mass mean molecular weight of 12000 and double bond content of 1 mmol/g. A curable composition comprising the obtained fluoropolymer had high curing rate and excellent productivity. A fluorine-containing cured material formed using curable composition had excellent light resistance, transparency, heat resistance, dimensional stability and productivity.