PURPOSE: To suppress the capture of hydrogen atoms to a defective part and to improve eventually the long-period stability and reliability of an optical fiber by incorporating fluorine into the core part and clad part respectively of the fiber and generating a difference in refractive index from the relative difference in the concentrations of fluorine in both parts.

CONSTITUTION: The fine glass particles formed by supplying SiCl4 into, for example, an oxyhydrogen flame and decomposing the same are deposited and grown axially to manufacture a porous glass body having a circular columnar shape. Such porous glass body is subjected to a heating treatment at 1,200°C in a gaseous helium atmosphere contg. thionyl fluoride SOF2 and is then heated up to 1,500°C in gaseous helium by which said body is made transparent and glassy. The pulverous glass particles formed as a result of the decomposition of the SiCl4 by the oxyhydrogen flame is deposited on the outside circumference of the quartz glass rod to form a glass layer for cladding. Such glass rod is subjected to a heating treatment to 1,500°C in a gaseous helium atmosphere contg. Si2OF6 by which the glass layer is melted and made transparent. The clad glass is thus formed. The refractive index of such clad glass layer is lower than the refractive index of the glass rod.