A high-frequency circuit system with a traveling wave tube (1, 2), the traveling wave tube (1, 2) comprising: an electron gun including a cathode for emitting electrons and a heater for giving heat energy for emitting electrons from the cathode, a helix in which an electron beam formed of said electrons emitted from the electron gun interacts with an RF, Radio Frequency, signal, a periodic magnetic field generation device for generating a magnetic field for suppressing the expansion of the electron beam passing through the helix, a collector for capturing the electron beam output from the helix, an anode for guiding the electrons emitted from the electron gun into the helix, and a magnetic field application device arrange for changing a magnetic field for changing the diameter of the electron beam, characterized in that the traveling wave tube is arranged to be operated at high power mode, at which the output power of the RF signal is maximum, or at a low power mode, at which the output power of the RF signal is low compared with the output power of the RF signal in the high power mode, and in that the high-frequency circuit system further comprises a magnetic field application power supply arranged for supplying electric power to the magnetic field application device for generating the magnetic field, the magnetic field application power supply being configured to supply at the time of a low power mode an electric power to the magnetic field application device that is smaller than the electric power supplied at the time of high power mode, in a case in which the traveling wave tube (1, 2) is designed so that the optimal trajectory of the electron beam can be obtained when the traveling wave tube (1, 2) operates in the high power mode, and to supply at the time of the high power mode an electric power to the magnetic field application device that is greater than the electric power supplied at the time of a low power mode, in a case in which the traveling wave tube (1, 2) is designed so that the optimal trajectory of the electron beam can be obtained when the traveling wave tube (1, 2) operates in the low power mode. an electron gun including a cathode for emitting electrons and a heater for giving heat energy for emitting electrons from the cathode, a helix in which an electron beam formed of said electrons emitted from the electron gun interacts with an RF, Radio Frequency, signal, a periodic magnetic field generation device for generating a magnetic field for suppressing the expansion of the electron beam passing through the helix, a collector for capturing the electron beam output from the helix, an anode for guiding the electrons emitted from the electron gun into the helix, and a magnetic field application device arrange for changing a magnetic field for changing the diameter of the electron beam, characterized in that the traveling wave tube is arranged to be operated at high power mode, at which the output power of the RF signal is maximum, or at a low power mode, at which the output power of the RF signal is low compared with the output power of the RF signal in the high power mode, and in that the high-frequency circuit system further comprises a magnetic field application power supply arranged for supplying electric power to the magnetic field application device for generating the magnetic field, the magnetic field application power supply being configured to supply at the time of a low power mode an electric power to the magnetic field application device that is smaller than the electric power supplied at the time of high power mode, in a case in which the traveling wave tube (1, 2) is designed so that the optimal trajectory of the electron beam can be obtained when the traveling wave tube (1, 2) operates in the high power mode, and to supply at the time of the high power mode an electric power to the magnetic field application device that is greater than the electric power supplied at the time of a low power mode, in a case in which the traveling wave tube (1, 2) is designed so that the optimal trajectory of the electron beam can be obtained when the traveling wave tube (1, 2) operates in the low power mode.