PURPOSE: To obtain a base material for an optical fiber without causing an abnormality in refractive index, by inserting a sootlike core having a higher refractive index into the interior of a sootlike cylindrical part.

CONSTITUTION: A sootlike clad cylindrical part 10 doped with SiO2 and a sootlike core 12 doped with GeO having a higher refractive index than that of the cylindrical part 10 are formed by the vapor-phase axial deposition (VAD) method. The outer peripheral surface of the core 12 is sized to give an outside diameter a little smaller than the inside diameter of the cylindrical part 10. The core 12 is then inserted into the cylindrical part 10 to form an assembly 14, which is then integrated. The resultant assembly 14 is then dehydrated in a chlorine atmosphere and finally heat-treated and vitrified. Thus, the refractive index distribution can be standardized even in both cases of graded index (GI) fibers and single mode (SM) fibers.