A communication system comprising a plurality of terminals, comprising: a start request information obtaining unit configured to obtain information for requesting start of a session between a first terminal and a second terminal of the plurality of terminals; a session management unit configured to establish a session between the first terminal and the second terminal in accordance with the start request information; characterized by: the start request information including designation indicating presence/absence of participation restriction for allowing/restricting a terminal, other than the first terminal and the second terminal, to/from participating in the session; a participation request information obtaining unit configured to obtain participation request information with which a third terminal of the plurality of terminals requests participating in the already established session that has already been established between the first terminal and the second terminal; and a participation determination unit configured to allow the third terminal to participate in the already established session when the start request information designates absence of the participation restriction, and to prohibit the third terminal from participating in the already established session when the start request information designates presence of the participation restriction, wherein when the participation determination unit allows participation in the already established session, the session management unit lets the third terminal to participate in the already established session. a start request information obtaining unit configured to obtain information for requesting start of a session between a first terminal and a second terminal of the plurality of terminals; a session management unit configured to establish a session between the first terminal and the second terminal in accordance with the start request information; characterized by: the start request information including designation indicating presence/absence of participation restriction for allowing/restricting a terminal, other than the first terminal and the second terminal, to/from participating in the session; a participation request information obtaining unit configured to obtain participation request information with which a third terminal of the plurality of terminals requests participating in the already established session that has already been established between the first terminal and the second terminal; and a participation determination unit configured to allow the third terminal to participate in the already established session when the start request information designates absence of the participation restriction, and to prohibit the third terminal from participating in the already established session when the start request information designates presence of the participation restriction, wherein when the participation determination unit allows participation in the already established session, the session management unit lets the third terminal to participate in the already established session. the start request information including designation indicating presence/absence of participation restriction for allowing/restricting a terminal, other than the first terminal and the second terminal, to/from participating in the session; a participation request information obtaining unit configured to obtain participation request information with which a third terminal of the plurality of terminals requests participating in the already established session that has already been established between the first terminal and the second terminal; and a participation determination unit configured to allow the third terminal to participate in the already established session when the start request information designates absence of the participation restriction, and to prohibit the third terminal from participating in the already established session when the start request information designates presence of the participation restriction, wherein when the participation determination unit allows participation in the already established session, the session management unit lets the third terminal to participate in the already established session.