A transmitting side communications device transmits a Delete Payload of an IPsec SA using an ISAKMP SA that satisfies a transmission condition, and more preferably using all ISAKMP SAs that exist between communications devices. The receiving side communications device accepts a Delete Payload by an ISAKMP SA that satisfies a reception condition, and preferably any ISAKMP SA that exists between the communications devices. As long as there exists an IPsec SA that was established using the ISAKMP SA, it remains possible to maintain an ISAKMP SA, without completely deleting it, and to transmit and receive a Delete Payload by that ISAKMP SA.