PURPOSE: To eliminate the need of any expensive oxide-impregnated cathode from an ion laser tube by constituting the cathode of the tube of a pair of closely arranged electrodes insulated from each other.

CONSTITUTION: Electrodes 11 and 15 are arranged in a state where they are insulated from each other at a fixed interval. The front end sections of the electrodes 11 and 15 are bent so that the end sections can face each other. A voltage for main discharge is applied across the electrode 11 or 15 and an anode 13 and main discharge is generated by the discharge between the electrodes 11 and 15. When the main discharge starts, secondary electrons are discharged from the electrode 11 or 15 due to ion collisions and the electrode 11 or 15 starts to exert the function of a cathode and the main discharge is maintained. Therefore, an inexpensive ion laser tube can be obtained, because the structure of the tube becomes simple and the tube does not require any expensive oxide-impregnated cathode.