PURPOSE: To uniformize the distribution of refractive index of an optical fiber preform in its longitudinal direction by fixing the flow rate of the GeCl4 in the exhaust gaseous Cl2 from the core tube of an electric furnace in sintering at the time of producing an optical fiber preform by VAD using a raw material contg. GeCl4.

CONSTITUTION: The raw materials such as SiCl4 and GeCl4 and a fuel such as H2 and O2 are introduced into a multiple-pipe burner, and the mixture is subjected to a reaction to produce a porous preform 4. The obtained porous preform 4 is arranged in the core tube 3 of an electric furnace, heated by a heater 5 while supplying gaseous He and gaseous Cl2, sintered, and vitrified. In this case, the flow rate of the GeCl4 in the exhaust gaseous Cl2 from the core tube 3 is detected by a mass flowmeter 9, the variation is fed back to the feed rate, temp., gas flow rate, etc., in the sintering conditions by a controller 10, and the flow rate of the GeCl4 in the exhaust gas is fixed. Consequently, an optical fiber preform with the amt. of GeO2 added uniformized in the longitudinal direction is obtained.