In the present invention, a start request terminal transmits start request information including participation restriction information to a management system. A transmission/reception unit of the management system transmits participation authentication information for authenticating participation in an established session to a middle-of-conference participation terminal. A transmission/reception unit of a participation request terminal transmits, to the management system, participation request information for requesting participation of the participation request terminal in an established session and participation authentication information which is input with the terminal in accordance with the participation authentication information transmitted to the terminal. A participation determination unit of the management system compares the participation request information received from the participation request terminal and the participation authentication information corresponding to the participating terminal in a terminal management table, and determines whether to permit the participation request terminal to participate in the established session.