Each terminal registers the key generation information into each session management server, the information including a plurality of setting items necessary for determining set values to generated a key to be used by itself, and set value candidates which are stored in the setting items. When the encryption communications are established between the terminals, the individual session management servers and a key generation information management server are associated, so that the key generation information management server selects the algorithm suite based on the key generation information. The session management server generates the parameters based on the selected algorithm suite, acquires the information on the selected algorithm suite from the key generation information management server, generates the key for the encryption communications based on that information and distributes the key to the each terminal.