PURPOSE: To obtain the titled material having low viscosity, superior adhesive strength to an optical fiber and a high curing rate and giving a flexible resin coat, by blending a specified epoxy resin with a salt of Lewis acid.

CONSTITUTION: An epoxy resin contg. aromatic, alicyclic or heterocyclic epoxy resin having two or more epoxy groups in the molecule such as bisphenol A type epoxy resin or aromatic or alicyclic glycidyl ester type epoxy resin as the principal component is blended with a salt of Lewis acid such as the diazonium salt of Lewis acid or the sulfonium slat of Lewis acid to obtan a coating materil for forming the coating layer of a coated body consisting of an optical glass fiber and the layer covering the fiber and having ≤10μm thickness and superior wear resistance. The coating mterial has low viscosity, superior adhesive strength to the optical fiber and a high curing rate, and the resulting resin coat is tough.