Abstract: An electron beam gas processing apparatus includes a single vacuum vessel maintained at vacuum by means of a vacuum pump and first and second lenses are disposed in the vacuum vessel. An electron beam emitted from an electron source is focused by each of the lenses, and the electron beam is irradiated onto a processing gas in a duct. When the current value of the electron beam is increased as the concentration of NOx in the processing gas increases, the focal distance is decreased by increasing the intensity of magnetic fields or the intensity of electric fields of the lenses in accordance with the current value of a filament, the current value of an arc power supply and the gas pressure in a gas reservoir and the first lens is moved toward a draw-out electrode and the second lens is moved toward the duct, so that a parallel electron beam of constant diameter is formed and the electron beam can be prevented from being increased in focusing diameter.