PURPOSE: To prevent the deterioration of the light transmitting performance by using a polymer contg. alkyl methacrylate as the core material of a heat resistant optical fiber usable at ≥80°C, coating the core material with a polymer contg. fluorine, and irradiating electron beams.

CONSTITUTION: A methyl methacrylate polymer or a copolymer of methyl methacrylate with a styrene deriv. is melted and spun by means of an extruder to form the core material of a plastic optical fiber. The outside of the core material is coated with a polymer contg. fluorine such as a copolymer of vinylidene fluoride with perfluoroolefin as the sheath material. By irradiating electron beams to the surface of the sheath material, the sheath material is cross-linked to form a plastic optical fiber with Tg 105°C heat resistance. The light transmission loss of the optical fiber is hardly increased even at a high temp. such as ≥90°C.