PURPOSE: To prevent permeation of external water and to obtain optical fiber generating small light transmission loss by forming silicon nitride film on the surface of an optical fiber core consisting of quartz, fluorinated Li, fluorinated glass, or fluorinated plastic.

CONSTITUTION: For example, an Si3N4 film 2 is formed continuously to about 1μm thickness on the surface of an optical fiber core 1 consisting of quartz, (SiO2), etc. by the CVD or PVD process. A plastic 3 having high tensile strength is covered on the surface of the film 2. By covering the surface of the optical fiber core 1 with Si3N4 film, as described above, the optical fiber core is prevented from increase of laser light transmission loss due to permeation of external water, moreover, such optical fiber generating extremely small light transmission loss for over several hundreds km length of the optical fiber, is provided.