PURPOSE: To raise the restraining effect of retrograde electrons and improve the efficiency of a traveling wave tube, while eliminating the influence of temperature over a permanent magnet by setting the permanent magnet on the surface of a supporting plate reverse to collector electrodes.

CONSTITUTION: A permanent magnet 14 is set on the surface of a supporting plate 2 reverse to collector electrodes 5W8. Namely, the magnet 14 is set on a portion of low temperature, so that the magnetic characteristic of the magnet 14 may not be affected by high temperature. Accordingly, a magnetic field having a radial component is generated on an electron orbit by magnetic flux generated in a collector can 4. Thereby, the restraining effect of retrograde electrons can be raised by curving the advancing direction of the retrograde electrons to the radial direction, and the efficiency of a traveling wave tube can be improved.