A disclosed mobile radio communications system comprises an exchange station; first and second radio base stations configured to radio-communicate with a mobile station and configured to relay data from the mobile station; first and second radio control stations configured to wire-communicate with the first and second radio base stations, respectively, and configured to establish a wired communication path for relaying the data transmitted from the mobile station to the exchange station; a transmission device configured to wire-communicate with the first and second radio base stations, and configured to establish a local wired communication branch for relaying the data transmitted from the mobile station between the first and second radio base stations without going through the first or second radio control station; a third radio base station configured to radio-communicate with the mobile station, configured to relay the data transmitted from the mobile station, and incapable of communicating with the transmission device; and a third radio control station configured to wire-communicate with the third radio base station, and configured to establish a wired communication branch for relaying the data transmitted from the mobile station to the exchange station.A method of adding a radio communication branch comprises the step of, when a request for adding a radio communication branch through the third radio base station is generated while the data transmitted from the mobile station is being relayed via the local wired communication branch, determining whether to reject the request, based on a radio quality parameter between the mobile station and the third radio base station.