a method for producing carbon fibers in a vertical carbonizing furnace and an apparatus for producing carbon fibers using such a method are disclosed . the furnace includes a heating chamber for carbonizing fibers , the furnace including , a fiber inlet at the upper end of the chamber an air tight sealed fiber outlet at the lower end of the furnace , an inert gas inlet provided on the wall of the chamber and above the fiber outlet , at least one inert gas injection portion , formed on the wall of the chamber , each capable of forming a curtain of inert gas across the heating chamber , each injection portion being provided between the gas inlet and the fiber inlet , at least one outlet each being provided below each inert gas injection portion , and a heating member capable of controlling the temperature in the heating chamber in such a manner that the temperature gradually increases from the upper end toward a lower end of the heating chamber . the carbon fibers produced by this method or apparatus are excellent in that they have few fluffs and cohering filaments and improved strength and ductility .