To obtain a radio communication system that clearly decides network unit of a radio terminal so as to simply designate a specific radio terminal as a repeater and to obtain a communication system where a network can easily be extended by using the repeater.

A radio terminal collects a role and sub network ID information of a radio terminal in existence within a communication available range of its own terminal. When no center station is in existence in a communication available range, the radio terminal discriminates that it itself is a center station (step 502), decides a sub network ID and informs surrounding terminals of it. When a plurality of radio terminals using different sub networks are in existence in the communication available range, the radio terminal discriminates whether or not a relay recognizing all the sub networks is in existence (step 508), and when it is not in existence, the radio terminal itself acts as a relay station (step 509) and informs all the existing radio terminals about the change in the role of the radio terminal (step 514). The extension of the network is realized by deciding the role of a new radio terminal according to such a procedure.