A travelling-wave tube amplifier comprises a travelling-wave tube, a plurality of collector electrodes, a plurality of collector leads and at least one magnetic core. The travelling-wave tube causes interaction between an electron beam and an input high frequency signal to amplify the high frequency signal, and outputs the amplified high frequency signal. The plurality of the collector electrodes capture the electron beam which was subjected to velocity modulation by the interaction. The plurality of the collector electrodes are electrically connected to a power source via the plurality of the collector leads. The at least one magnetic core comprises a through hole. Any two of the plurality of the collector leads go through the through hole in one of the magnetic cores. Currents flowing through the two collector leads through the through hole in the magnetic core generate magnetic fluxes in the magnetic core. The directions of the generated magnetic fluxes are reversed from each other.