PURPOSE: To decrease the number of stand-by radio devices by providing a common stand-by radio device for plural in-use radio devices.

CONSTITUTION: The stand-by radio device 3 which is used in common among in-use radio devices A∼(n) is provided to radio stations where the devices A∼(n) are installed. If a fault occurs to one of the devices A∼(n), it is switched automatically to the stand-by radio device 3.