PURPOSE: To obtain the titled covering material capable of forming a film with superior flexibility at a high curing speed by blending hydrogenated 1,2-polybutadiene having hydroxyl groups with an isocyanate compound.

CONSTITUTION: Hydrogenated 1,2-polybutadiene (A) having two or more hydroxyl groups and ≥about 50% rate of hydrogenation is blended with a compound (B) having two or more isocyanate groups such as tolylene diisocyanate so that the isocyanate groups in the component B is contained by about 1W1.1 equiv. per 1 equiv. hydroxyl groups in the component A. A curing catalyst (C) such as dibutyl tin dilaurate may be added. A covering material for an optical glass fiber is obtd.