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

CONSTITUTION: A coating material obtd. by blending an epoxy resin contg. ≥4C aliphatic epoxy resin having two or more epoxy groups in the moelcule such as long-chain aliphatic diglycidyl ether of 1,4-butanediol or diglycidyl ether of polyether polyol having ≥200 average mol.wt. as the principal component with a salt of Lewis acid such as the diazonium salt of Lewis acid is used as a material for forming an intermediate layer which is more flexible than a surface layer between an optical glass fiber and the surface layer covering the fiber and having superior wear resistance such as a polyethylene or polyamide resin layer. The coating materil has low viscosity, superior adhesive strength to the optical fiber and a high curing rate, and the resulting resin coat is flexible.