To provide an electron accelerating device having a long lifetime for nearly eternal use and having the structure not requiring the ordinary heating of a fixed cathode by a heater and capable of using the reversely flowing electron eB.

Electron drawn from an electron generating cathode is accelerated by utilizing the acceleration energy obtained from an acceleration electric field generated in an acceleration cavity during the time of passing of the electron in the acceleration cavity. At this stage, the electron generating cathode injects the gas to a plasma generating part with a timing of the electron acceleration so as to generate the plasma P1, desirably, the pulse-like plasma P1, and electron is drawn from the plasma P1 into the acceleration cavity 1. Furthermore, a plasma cathode Pc is arranged in both sides in the accelerating axial direction of the acceleration cavity 1 so that the electron drawn from the cathode can enter at all phase of the accelerating electronic field inside of the acceleration cavity 1.