Linked with a position registration procedure in a mobile IP, the invention provides a VPN setting service using an IP Sec. tunnel between optional terminals without requiring these terminals to have a specific VPN function. This service is provided by a mobile terminal, authentication servers, a VPN database, and network apparatuses. A home authentication server extracts from the VPN database the VPN information of a user who has requested the authentication at the time of making a position registration request from the mobile terminal. The home authentication server then posts the VPN information to each network apparatus using a predetermined position registration message and an authentication response message. Based on the posted VPN information, the network apparatuses set a VPN path by the IP Sec. to between a home network apparatus and an external network apparatus, between the home network apparatus and a predetermined network apparatus, and/or the external network apparatus and the predetermined network apparatus, respectively.