PURPOSE: To provide the flexible optical fiber bundle having improved moisture resistance at a low cost by coating the surface of a clad material with frit glass.

CONSTITUTION: The surface of the clad material 2 of the optical fiber formed by using multicomponent type glass for the clad material 2 is coated with the frit glass 3. Compsns. basically consisting of B2O3-PbO-ZnO, B2O3-PbO-SiO2, etc., are easily usable as the compsn. of the frit glass 3 in terms of the good moisture resistance and adjustability of the m.p. and coefft. of thermal expansion over a wide range by additives. The method for coating the surface of the clad 2 with the clad glass 3 includes a method of dispersing the powder of the frit glass 3 into a solvent contg. a binder to prepare paste and applying this paste onto the clad, then removing the binder by baking, thereby forming the thin and dense frit glass layer 3 on the surface of the clad.