PURPOSE: To select arbitrary combination from among living and spare transmitter-receives in the case of looping back a circuit test signal in the digital radio transmitter-receiver of a hot standby system.

CONSTITUTION: In the case that a line loop back test is executed, by changeover- controlling test signal returning base band switches 5 and 6 connected respectively between an active/standby changeover base band switch 22 and a receiver output terminal 12 at a receiver side and on the other hand, between a transmitter input terminal 13 and a base band signal branch circuit 23 at a transmitter side so that the signal is returned from the receiver to the transmitter by a control signal from the outside, and further, by changeover-controlling the active/standby changeover base band switch 22 at the receiver side and an active/ standby changeover high frequency switch 30 at the transmitter side respectively by using the control signal, the test signal can be looped back from any living or spare receiver through any living or spare transmitter.