PURPOSE: To obtain a long-period reliability of characteristics such as heat resistance and environmental resistance by incorporating a multifunctional reacting agent in a polymer in a bulk polymerizing process or fusion spinning process and heating or irradiating a fusion-spinned fiber with radiation.

CONSTITUTION: This manufacture consists of the bulk polymerizing process 3 for monomers 1 and 1' which produce an amorphous polymer or copolymer and the process 5 in which fusion spinning is carried out while remaining volatile components are removed from the obtained polymer; and the multifunctional reacting agent 6 is added to and incorporated in the polymer in the bulk polymerizing process 3 or fusion spinning process 5 and the fiber after the fusion spinning is heated or irradiated 13 with radiation. The polymer of the core of this obtained plastic optical fiber has cross-linked or entangled structure by reacting on the multi-functional reacting agent after spinning and drawing, so the orientation of the core polymer is not even when the fiber is heated above glass dislocation temperature. Thus, the fiber has superior thermal characteristics and small temperature dependency of optical transmission loss. Further, the multifunctional reacting agent is made omnipresent near the periphery of the core to provide a refractive index distribution, thereby improving the transmissivity.