Abstract: In a digital radio communication system using at least two regular channel and two or more standby channels for transmitting a test signal therethrough in the standby condition thereby permitting the regular channel to be synchronously switched to one of the standby channels, the improvement wherein a channel switching system for monitoring the standby channels is provided at the receiving side of the digital communication system with: control signal generator means operating so that when two or more standby channels are in standby condition, it generates a control signal to designate them in turn, and when only one standby channel is in standby condition, it generates a control signal to designate the channel; synchronizing switching means for selectively outputting the test signal transmitted via each of the standby channels designated by the control signal; and standby channel monitoring means for detecting an output from the synchronizing switching means to monitor the communication quality of the standby