PURPOSE: To obtain high heat resistance, bending and flexibility by using a transparent resin having specified or higher thermal deformation temp. for a core material.

CONSTITUTION: The transparent resin used for the core material has ≥120°C thermal deformation temp. As for the transparent resin, for example, an optical fiber comprising a heat-resistant acryl resin, thermosetting silicone resin or thermoplastic norbornene resin can be used. A heat-resistant acryl resin to be used has ≥120°C thermal deformation temp., and preferably ≥140°C, and for example, it can be produced by crosslinking and hardening cross-linkable (meth)acryl monomers or (meth)acryl monomers containing cross-linkable (meth) acryl monomers by heat or light. The cross-linkable monomers are, for example, polyfunctional (meth)acryl monomers such as diethylene glycol diacrylate.