To provide a cavity-coupled traveling wave tube which has an excellent heat radiation in a slow-wave circuit and an enhanced yield of assembling.

In a cavity-coupled traveling wave tube which has a slow-wave circuit part 12 where a plural of cavity components 12b are arranged, an evacuated envelope 17 which is composed of magnetic pole pieces 17a and spacers 17b and surrounds the outside of the slow-wave circuit part 12, and flanges 14, 15 to which magnetic pole pieces 17a of the evacuated envelope 17 are connected, the evacuated envelope 17 is composed of a plural of unit parts U1-U4 in which magnetic pole pieces 17a are connected mutually and the length of unit parts U1, U4 which is connected to the flanges 14, 15 is shorter than that of the unit parts U2, U3 which is not connected to the flange.