In order to reduce the frequency with which communication occurs when updating a network key is reduced and minimize the deterioration in performance due to updating without relying on a key tree, a server is provided with an address key allocation unit which generates identifiers for identifying clients by the combination of addresses on a plurality of address spaces and allocates address keys to respective addresses included in the generated identifier, and a network key ciphering unit which generates a network key update key which cannot be generated from the address keys allocated to a client to be disconnected, ciphers a new network key using the network key update key, and delivers the new network key to the clients.