PURPOSE: To manufacture an optical fiber superior in heat resistance and light transmission performance by coating a core component composed essentially of a specified deuterium-substd. (poly)methyl methacrylate with a sheath component substance made of a fluorine-contg. polymer.

CONSTITUTION: A deuterium-substd. (poly)methyl methacrylate type copolymer is obtained by adding 0.0002W0.1mol% radical polymn. initiator, such as dicumyl peroxide, 0.01W10mol% chain transferring agent, such as tert-butyl mercaptan, and 1W50wt% nonpolymerizable org. solvent, such as methanol, to deuterium- substd. methyl methacrylate or a mixture of it and other copolymerizable monomer, introducing them into a continuous polymerization vessel, polymerizing them at 50W200°C, and heating them at 150W300°C under 10W100kg/cm2 pressure to remove volatile matters, such as unreacted monomers and the org. solvents. Then, this copolymer is spun to form a core component, and this core component is coated with a sheath component made of a fluoropolymer contg. ≥20wt% fluorine.