A transmitting/receiving unit receives a SIP signal after occurrence of trouble in a SIP server and outputs a call ID of the SIP signal to a recovery-file searching unit. A session control unit once again procures a call process resource and an instance for a session corresponding to a recovery file and stores the call process resource data and the instance data in a main storage unit. The main storage unit stores anew session data included in the recovery file. A recovery-file creating unit creates the recovery file. An external storage unit stores therein the recovery file. The recovery-file searching unit retrieves from the external storage unit the recovery file that matches with the call ID output by the transmitting/receiving unit and outputs the recovery file to the session control unit and the main storage unit.