A resin-made heat-resistant optical fiber is formed of a core of a hardened thermosetting resin and a cladding of a polymer or copolymer having a refractive index smaller by at least 1%, preferably, at least 3% than that of the core. The thermosetting resin is selected from the group consisting of epoxy resins, phenol resins and urethane resins. The polymer or copolymer may preferably be a thermoplastic organic resin.
