To reduce the pressure loss of electron guns, the number the electron guns, and costs by providing an electron beam transmission window and an anode at a plurality of positions of the outer-enclosure part of a cylindrical vacuum container and arranging a cathode unit being provided so that it is paired with the anode on the center line of the container.

Electron beam transmission windows 2 and 3 are provided at the outer-enclosure part of a vacuum container 1 while being separated and a pair of electrodes 4 and 5 are provided inside in parallel. A cathode unit 8 consisting of two cathodes 6 and 7 being provided so that they can be paired with the anodes 4 and 5 is arranged at nearly the center in the container 1. Electrons beams 13 are emitted from the recessed part of the unit 8, pass between two parallel anodes 4 and 5, and are discharged into a gas through the transmission windows 2 and 3. The beams 13 are discharged in two directions symmetrically with a symmetrical surface 14. The beams 13 are emitted in two opposite directions against a gas flow from one electron gun, thus increasing the interval of the electron guns and reducing a pressure loss, reducing the number of the electron guns, and reducing costs.