A radio terminal (10) and a radio relay (20, 30, 40) perform radio communications therebetween by a radio communications system else than a TDMA system, an interference measurer (13a) measures a degree of interference of each of radio slots constituting a radio channel employed in the radio communications system, a decision maker (13b, 13c) makes a decision, depending on a measurement result of the degree of interference, whether or not the radio communications by the radio communications system is able to be continued, and a switcher (42) switches, as the decision is made of the radio communications by the radio communications system to be unable to be continued, the radio communications system to a radio communications system of the TDMA system.