A private branch exchange system comprising: a radio communication terminal capable of executing direct communication with another radio communication terminal without involving a radio access network; and a private branch exchange server configured to provide a communication service via the radio access network to the radio communication terminal belonging to an extension group, wherein the private branch exchange server includes a transition trigger detection unit configured to detect a trigger for a transition to the direct communication, a list retention unit configured to retain an extension group list containing identifiers of the radio communication terminals belonging to the extension group, a subgroup selection unit configured to select a certain subgroup from the extension group list, and a direct communication command unit configured to send permission information on the direct communication and configuration information on a communication environment necessary for the direct communication to the radio communication terminals contained in the subgroup selected by the subgroup selection unit, and the radio communication terminal includes a direct communication execution unit configured to execute the direct communication based on the permission information and the configuration information received from the private branch exchange server. a radio communication terminal capable of executing direct communication with another radio communication terminal without involving a radio access network; and a private branch exchange server configured to provide a communication service via the radio access network to the radio communication terminal belonging to an extension group, wherein the private branch exchange server includes a transition trigger detection unit configured to detect a trigger for a transition to the direct communication, a list retention unit configured to retain an extension group list containing identifiers of the radio communication terminals belonging to the extension group, a subgroup selection unit configured to select a certain subgroup from the extension group list, and a direct communication command unit configured to send permission information on the direct communication and configuration information on a communication environment necessary for the direct communication to the radio communication terminals contained in the subgroup selected by the subgroup selection unit, and the radio communication terminal includes a direct communication execution unit configured to execute the direct communication based on the permission information and the configuration information received from the private branch exchange server. a transition trigger detection unit configured to detect a trigger for a transition to the direct communication, a list retention unit configured to retain an extension group list containing identifiers of the radio communication terminals belonging to the extension group, a subgroup selection unit configured to select a certain subgroup from the extension group list, and a direct communication command unit configured to send permission information on the direct communication and configuration information on a communication environment necessary for the direct communication to the radio communication terminals contained in the subgroup selected by the subgroup selection unit, and the radio communication terminal includes a direct communication execution unit configured to execute the direct communication based on the permission information and the configuration information received from the private branch exchange server.