To provide a traveling-wave tube capable of eliminating a delay circuit, making an effective band extremely wide, and easily obtaining a large amount of output, with a simple structure using a waveguide of a TM mode as a coupled circuit.

In this traveling-wave tube in which an electron gun to discharge electron beams, an acceleration device to accelerate the electron beams discharged from the electron gun, a radiowave input device to which radiowaves such as microwaves and millimeter waves are input, a radiowave output device to output amplified radiowaves, and a positive electrode to catch the electron beams discharged from the electron gun are disposed in a vacuum vessel, the waveguide in conformity with a TM mode is disposed between the radiowave input device and the radiowave output device to propagate radiowaves of the TM mode, and the electron beams can thereby travel in the waveguide. The speed of the electron beams is in a relativistic speed range.