A digital radio relay system has a control station having a line control unit, the control station having at least a first mobile station located in its communication zone, and a relay station coupled to the control station by a radio channel, the relay station having at least a second mobile station located in its communication zone, so that the control station and the relay station can be communicated by the radio channel, wherein when the first and second mobile stations communicate with each other, the line control unit has a timing control unit for adjusting timing of a communication between the first and second mobile stations, and the timing control unit adjusts a difference between a line connection time of the control station and the mobile station in the control station zone and a line connection time of the control station and the mobile station in the relay station zone.