The present invention is directed to a heating furnace for heating a porous preform made of fine particles of quartz base glass to form an optical fiber. The furnace comprises a heater and a muffle tube body positioned inside the heater to separate the heating atmosphere from the heater. The muffle tube body consists of highly pure carbon, and inner and outer walls of the body are coated with pyrolytic graphite or a solid-phase carbonized glassy carbon. As a result of this, the muffle tube does not deteriorate even if a corrosive gas at high temperature is present in the furnace.