PURPOSE: To form a cladding which is plenty in the doped amount of fluorine and made of the quartz glass for a short time by bringing fluorine-contg. Lewis acid into contact with the soot of SiO2, heating and vitrifying it.

CONSTITUTION: The soot of SiO2 is formed from a gaseous mixture contg. SiCl4 and O2 or the like. Fluorine-contg. Lewis acid such as BF3 is brought into contact with the soot at ≤800°C. The soot of SiO2 added with the fluorine- contg. Lewis acid is heated in the inert atmosphere and vitrified to form a cladding. In this way, the base material for a quartz glass series optical fiber consisting of both the cladding of quartz glass doped with fluorine and the pure quartz glass is obtained.