PURPOSE: To reduce the fluctuation in refractive index and obtain a base material for optical fibers having a suitable refractive index distribution, by adding a refractive index controlling additive and further a diffusion accelerating additive to a raw material gas in producing the titled base material by the vapor-phase axial deposition (VAD) method.

CONSTITUTION: A raw material gas blown from a multiple tube burner is subjected to flame hydrolysis to form fine glass particles. In the process, GeCl4 and CCl2F2 as a diffusion accelerating additive are added to the above-mentioned raw material gas SiCl4, and the resultant mixture gas is then subjected to the flame hydrolysis in a flame of an oxyhydrogen burner to form the fine glass particles, which are then deposited on the tip of a rotating starting base material to grow a porous base material in the axial direction. OH groups in the resultant base material are then removed with chlorine, and the base material is heated at a specific temperature in an electric furnace to give a sintered transparent glass, and the aimed base material for graded index (GI) type optical fibers is produced.