PURPOSE: To obtain the titled base material having a satisfactory refractive index distribution and superior performance by heating a porous base material in the core tube of an electric furnace while moving the base material from the upper part to the lower part and feeding an inert gas and halogen from the lower part to the upper part so as to remove well OH.

CONSTITUTION: Gaseous starting materials are blown into a hot oxyhydrogen flame from a burner to produce SiO2-GeO2(-P2O5) type fine oxide powder, and this powder is deposited on the outside of a target 1 to form a porous base material 3. This base material 3 is put in the core tube 2 of an electric furnace so that it is positioned above a heater 4 placed around the core tube 2. The core tube 2 is made of quartz glass. The base material 3 is then heated with the heater 4 to form transparent sintered glass 5, and a base material for an optical fiber is obtd. At this time, the base material 3 is moved downward under rotation, and a gaseous mixture of gaseous He with a little gaseous Cl2 is introduced into the core tube 2 from the inlet 6 and exhausted from the outlet 6' so as to keep the internal pressure of the core tube 2 slightly higher than atmospheric pressure.