PURPOSE: To obtain an electron beam having plural kinds of energies by a small accelerator by moving an accelerating cavity in a uniform magnetic field, and dispensing with a deflecting pipe.

CONSTITUTION: An accelerating cavity 1 is installed in a line stretcher 4 of a wave guide whose length is variable, and can be moved in a radius vector direction of a pole plate 2. An electron (e) emitted from an electron gun 5 is made incident in the accelerating cavity 1 from a beam passing hole 6. The incident electron (e) is accelerated by a high frequency accelerating electric field E created in the cavity 1, and is emitted outside of the cavity 1 from an opposite side passing hole 6. The emitted electron (e) makes circular orbital motion in a magnetic field B, and is made incident again in the cavity 1, and is accelerated further by the electric field E, and makes larger circular orbital motion, and is made incident again in the cavity 1. This operation is repeated, and the electron (e) increases its energy, and a diameter of the circular orbital motion becomes large. The electron (e) accelerated to desirable energy is taken outside by a taking-out pipe 7 arranged on the circular orbit.