A terminal device in a server-client system including N terminal devices, a key distribution management device, and an authentication information database, where N is an integer greater than 1, the terminal device comprising: a list/request sending unit that, when the terminal device operates as an owner device, generates a key distribution request that contains a user list containing IDs of terminal devices participating in a session and an ID of the terminal device as the owner device, signs the key distribution request with a secret key of the terminal device as the owner device, transmits the key distribution request to the key distribution management device, and maintains a connection with the key distribution management device; a participation request sending unit that, when the terminal device operates as a general device, generates a participation request containing the ID of the terminal device, signs the participation request with the secret key of the terminal device, transmits the participation request to the key distribution management device, and maintains the connection with the key distribution management device; a session key generating unit that executes an authentication-based multipoint key distribution algorithm of server-client type in cooperation with another terminal device participating in the session and with the key distribution management device to generate a session key; and a post-confirmation unit that, when the session key has been normally generated, transmits a success notice indicating that the session key has been normally generated to the key distribution management device, and receives, from the key distribution management device, a collective notice meaning that the success notice has been received from all of the terminal devices participating in the session. a list/request sending unit that, when the terminal device operates as an owner device, generates a key distribution request that contains a user list containing IDs of terminal devices participating in a session and an ID of the terminal device as the owner device, signs the key distribution request with a secret key of the terminal device as the owner device, transmits the key distribution request to the key distribution management device, and maintains a connection with the key distribution management device; a participation request sending unit that, when the terminal device operates as a general device, generates a participation request containing the ID of the terminal device, signs the participation request with the secret key of the terminal device, transmits the participation request to the key distribution management device, and maintains the connection with the key distribution management device; a session key generating unit that executes an authentication-based multipoint key distribution algorithm of server-client type in cooperation with another terminal device participating in the session and with the key distribution management device to generate a session key; and a post-confirmation unit that, when the session key has been normally generated, transmits a success notice indicating that the session key has been normally generated to the key distribution management device, and receives, from the key distribution management device, a collective notice meaning that the success notice has been received from all of the terminal devices participating in the session.