PURPOSE: A material for optical fibers in which a plastic clad layer is provided on the outside of the core glass that is composed of natural molten quartz glass and synthetic quartz glass, thus reducing the fluctuation of transmission loss.

CONSTITUTION: The core part of the fiber is composed of 2 layers of synthetic quartz glass 10 and natural, molten quartz glass 11 and the outside of the core part is coated with a silicone resin as a the clad material. A quartz rod for optical fiber is prepared by synthesizing a quartz glass with low transmission loss on the inner, outer, or both wall surface of the molten quartz glass pipe by chemical vaporization. The rod is spun into fibers, which are simultaneously coated with a plastic clad as silicone resin. The ratio of Vo/V, where V is the volume of all of the glasses and Vo is the volume of glass synthesized by chemical vaporization is made in the range from 1/104 to 1/4, thus controlling the transmission loss of the fibers.