PURPOSE: To reduce a line instantaneous cut-off rate due to fading, by making a receiving side and a transmitting side have a space diversity function, in an FM microwave radio communication system of a setting stand-by system of space diversity.

CONSTITUTION: In a station A, an output from a transmitter 1 or 1' through a changeover switch SW4 is inputted to a changeover SW15, and is connected to an antenna 10 through an antenna sharing device 3. In a station B, inputs from antennas 10', 11' are inputted to receivers 2", 2''' through antenna sharing devices 3', 16', respectively, so as to have a space diversity function. In case when an output level of the receivers 2", 2''' of the station B has become a prescribed threshold level or below due to fading FD, an IF level detecting circuit 13 detects it, and sends out an alarm to the station A from an alarm sending-out circuit 12'. In the station A, when the alarm is detected by an alarm detecting circuit 14, the changeover SW15 is operated, and the output of the transmitter 1 or 1' is transmitted from an antenna 11 whose position is different from the antenna 10 through an antenna sharing machine 16. In this way, the station B is capable of receiving a signal without being influenced by FD.