To provide a radio communication system having: radio base station devices 111, 112, radio mobile terminal devices 121 to 124; and a group communication server 101, to efficiently start group communication.

A communication path is connected between the radio base station devices and the group communication server for each of a plurality of IP addresses unallocated to the radio mobile terminal devices. The radio base station device detects the radio mobile terminal device, and allocates the unallocated IP address to notify identification information and the IP address of the radio mobile terminal device to the group communication server, and the group communication server stores association between the identification information and the IP address. The radio mobile terminal device transmits by radio request to start group communication to the radio base station device, the radio base station device transmits the received request to start group communication to the group communication server, and the group communication server specifies the IP address of the radio mobile terminal device to establish the group communication.