PURPOSE: A glass tube in which an optical fiber preform is coaxially set is made to rotate, while an etching gas containing a fluoride is fed into the glass tube, and the preform is heated with a moving heat source outside the glass tube to remove the defects on the surface and the foreign substances of the preform uniformly.

CONSTITUTION: The glass tube 4 in which the above preform 1 is coaxially set is fixed to a pair of chucks 5, 5 in the glass lathe rotatably. While the pipe is being made to rotate by the glass lathe, the oxyhydrogen burner 8 is burned and reciprocated. Then, a fluoride gas and steam are fed together with a helium gas and/or an oxygen gas into the tube 4 to effect the etching of the glass on the surface of the preform for removing the defects and foreign substances on the surface. Then, steam is stopped and the fluoride gas and the helium gas are made to flow inside the pipe 4. When the fluoride is, e.g., SiF6, the HF gas, which is formed by the reaction thereof with steam, etches the preformed surface to convert the SiO2 in the etched glass into SiF4, which is vaporized off.