In a proxy mobile IP system, a mobility anchor comprises: a proxy mobile IP registration unit effectuating proxy mobile IP registration in proxy mobile IP (Internet Protocol) communication by one or more registration notification messages from an access gateway; and a last registration time recording/management unit effectuating recording and management of time of last registration by the registration notification message. The mobility anchor, on receipt of a new registration notification message, determines, in case a receipt time difference (or a receipt time interval) between the time of receipt of the new registration notification message and a registration notification message recorded last time in the last registration time recording/management unit is within a pre-set period of time, and the access gateway has changed by switching, possibility of order reversal of the new registration notification message and the registration notification message recorded last time.