A combustion furnace includes an inner shell, an outer shell, a gas inlet piping and a flame inhibiting cover. The inner shell defines a receiving cavity therein. The inner shell defines a plurality of first gas holes around the periphery of a top portion thereof. The inner shell defines a gas inlet hole at a bottom thereof. The outer shell encloses the inner shell such that a gas flowing space is defined between the inner shell and the outer shell. The gas inlet piping has an opening formed at one end thereof, and the gas inlet piping communicates with the gas flowing space. The flame inhibiting cover is atop the outer shell and the inner shell, and a lower flange of the flame inhibiting cover is below the first gas holes.