PURPOSE: To prevent oscillation in a high impedance region in a high frequency circuit of a traveling wave tube by making a resonance frequency of a cavity having loss members equal to that of a cavity having a dielectric so as to accord with a passing band of a high frequency circuit.

CONSTITUTION: A dielectric 4 made of ceramic or the like is housed inside of a cavity 2 so that a resonance frequency of the cavity 2 without a loss button accords with a resonance frequency of a loss button cavity 1 having oscillation preventing loss buttons 3. Consequently, a passing band of a high frequency circuit composed of the loss button cavity 1 is the same as that of a high frequency circuit formed of the cavity 2, thus preventing oscillation in a high impedance region.