PURPOSE: To prolong the service life of a carbon heating element by arranging the element outside a quartz furnace core tube, surrounding the element with a gaseous N2 atmosphere, electrically heating the element and forming an Si3N4 film on the element surface.

CONSTITUTION: A quartz glass fine-particle preform 7 is inserted into a quartz furnace core tube 1 with a carbon heating element 3 arranged outside, and 80 inert gas such as He is introduced from an inlet 2. Meanwhile, gaseous N2 is introduced from an inlet 8 into a furnace casing 4 surrounding the element 3 to form a gaseous N2 atmosphere. A specified power is then supplied to the element 3 to heat the inside of the tube 1 to about 1500°C, and the preform 7 is vitrified. Consequently, Si vapor from the tube 1 reacts with the gaseous N2 to form an Si3N4 film 9 on the element 3 surface, and the element is protected from oxidation.