A traveling-wave tube is provided which has a heat radiating structure with good heat conductivity from the collector core to the heat sink. An insulator is disposed around and in contact with the outer peripheral surface of a collector electrode and has a slit in parallel with a tube axis. A radiator is disposed outside of and in contact with the insulator and has a slot formed on the top surface thereof extending in parallel with the tube axis. The radiator also has one or more grooves that insect the slot and receive therein a fastening member having a length shorter than the length of the groove and formed with screwed holes extending through both end portions thereof in the axial direction. By adjusting amounts by which bolts are screwed into the fastening member through the throughholes formed through the radiator, the gaps in the groove between the radiator and both end surfaces of the fastening member can be varied. This allows a fastening force to be transmitted from the radiator to the entire insulator, thereby readily adjusting the degree of contact between the radiator and insulator and collector electrode.