A 3G authentication apparatus includes: a WLAN communication section which receives a notification of reception of a connection request sent from a WLAN access network in response to the connection request from a mobile unit to connect to a WLAN capable of interworking with a mobile network; a communication section for obtaining, from a user information storing device, information indicating a service available in the mobile communication network to the mobile unit that has sent the connection request, on the basis of the notification of reception of the connection request; and a filter setting section which generates filter information by associating a user authentication ID identifying a mobile unit with an IP address obtained at the communication section. The filter information generated by the filter setting section is sent to the WLAN access network through the WLAN communication section. Thus, traffic in a network such as a WLAN in an interworking system between networks such as a mobile communication network and a WLAN can be reduced.