To provide a digital radio system that prevents a sound that is hard to hear from being output from a terminal device that receives and demodulates frames even if data in the frames are delayed or missing.

In the digital radio system including a radio relay device wherein two of first and second radio base station devices are connected via a communication line, one of the radio base station devices includes a means that receives and demodulates a digital radio signal transmitted from a first mobile communication terminal and transfers the obtained digital data to the other radio base station device via the communication line, while the other radio base station device includes a means that resends a frame signal including a rebuilt synchronization word as a digital radio signal to a second mobile communication terminal. The second mobile communication terminal performs a repetitious operation of a previously obtained normal demodulation sound when it receives an IDLE frame first, and performs a mute operation when IDLE continues for a predetermined number or more of multiple frames.