A communication system has a plurality of nodes that perform encrypted communication via a LAN, each using an identical common cipher key. The common cipher key is replaced at fixed or irregular intervals, by being transmitted from a main node in a broadcast mode via the LAN to respective secondary nodes that are to share the key. When the key is successfully received by a secondary node, it returns a confirmation signal. The system can be configured such that a notification list of secondary nodes for which key acquisition has been confirmed is transmitted to all of the secondary nodes.