PURPOSE: To attain highly reliable switching operation into a standby unit shifting the resonance frequency to a frequency band not tuned to the output of the frequency of all transmitters connected to a common unit without any external control signal and keeping the tuning frequency in the said frequency band.

CONSTITUTION: A resonator control circuit 13, based on measured data 30, 31, applies tuning operation when an incident power is within a rage of a preset transmission power to control a tuning frequency variable type resonator 12 thereby maximizing the ratio of the incident/reflected power. Moreover, the resonance frequency is shifted to a frequency band not tuned to the output of the frequency all of the transmitters connected to the antenna multicoupler at the outside of the range and the tuning is kept in the band. Thus, the highly reliable and simple changeover of the standby transmitter is attained if a transmitter is faulty during the operation.