PURPOSE: To attain highly reliable changeover of a standby transmitter in case of the occurrence of a faulty transmitter by measuring a transmission power or frequency of a transmitter to be connected and applying automatic tuning to the transmission frequency when the measured value is within a preset range and keeping the tuning frequency at the outside of the operating frequency in an antenna multicoupler at the outside of the range.

CONSTITUTION: If an active transmitter 1 is faulty and an abnormal transmission wave is inputted to an automatic tuning resonator 63, a resonating circuit 63 recognizes it automatically that the transmission power or frequency exceeds a specified range and enters the tuning at a frequency band not tuned to all transmission frequenies connected to an antenna multicouple. In this case, it is possible for a control circuit 62 to recognize the abnormity in the transmitter 1 based on a communication signal 271. When the control circuit 62 judges a fault by a control signal 251 from the transmitter 1 even within the specified range, the transmission is stopped and an automatic tuning resonator 63 enters the tuning at the frequency band not tuned to all the transmission frequencies connected to the antenna multicoupler. Thus, the highly reliable switching to the standby transmitter is attained.