PURPOSE: To reduce the power consumption and to prevent adverse effect on the life of a traveling tube, by taking the anode voltage of the traveling wave tube of a spare device to 1/2∼1/3 of a voltage operated as a common use device.

CONSTITUTION: A switching control section 8 controls in such a way that a device 1 of traveling wave large power amplifiers 1, 2 is taken as a common use and its high frequency output is outputted to an antenna 5 and a signal taking the anode voltage as a steady-state value is given to a control circuit 3 of the device at the same time. On the other hand, the device 1 does not give an anode voltage control signal to the device 2 being the spare device and the anode voltage is controlled to a low value being 1/3∼1/4 of the steady-state value. Since the anode voltage is 1/3∼1/2 of the steady-state value of the spare device, then the adverse effect on the life of the traveling tube is prevented and the power consumption is reduced.