PURPOSE: To prevent a difference of output power for each operating frequency from being produced by providing a control means operated by the same transmission power as transmission power decided for a faulty radio transmitter.

CONSTITUTION: A transmitter control circuit 12 of a standby radio transmitter 2 receiving a transmitter changeover instruction sent from an automatic control circuit receiving an alarm signal informing of the occurrence of a fault gives a changeover instruction to a radio frequency predetermined for the faulty FM radio transmitter and a voltage changeover instruction to a voltage corresponding to transmission power to a frequency changeover control circuit 11 and a voltage changeover control circuit 10 respectively. The frequency changeover control circuit 11 controls a synthesizer of an FM exciter 6 to present it that an output frequency is the same as a radio frequency of the faulty active transmitter. Then a frequency of a standby radio transmitter is switched to the same frequency as the faulty active transmitter and the transmission power is switched to the same output power as that of the faulty active radio transmitter.