In switching of a conference server in a group conference system, switching of a session of each terminal device is completed smoothly in as a short time period as possible without imposing loads on a call control server. A conference server 121 connected to a call control server 110 which relays a request for call control of a group conference system and a plurality of terminal devices 130, including a session shift control unit 223 which determines an order of a request for call control of each terminal device which request is related to cut-off from a target from which a session is to be shifted and connection to a session shifting destination according to information related to session generation of each terminal device and a conference condition and determines timing of call control based on a load condition of the call control server, and a session shift instructing unit 224 which transmits a request for call control to the call control server according to the determined order and timing.