To satisfy both security of data communication by the IPsec/IKE protocol and convenience of data communication by seamless roaming.

An IKE cipher communication path in parallel with an IPsec cipher communication path is utilized and when an IP address is modified, updating of the IPsec cipher communication path is requested from the modified IP address to a communicating party by an IKE phase-2 protocol. At an opposite party side, as long as identity of a request source can be confirmed from identity as a prior cookie, information required for the updating is negotiated by the IKE protocol in response to the request regardless of the difference of the IP address. The IPsec cipher communication path based on the new address is created between both communicators and the former communication path is deleted.