In a server, an echo-request transmitting unit 204 transmits echo-request data to a target device, and an echo-reply receiving unit 205 receives echo-reply data from the target device. A time measuring unit 206 measures, as the target time, the time required between transmission of the echo-request data and reception of the echo-reply data, and compares the target time with the reference time. In this way, the server judges whether the target device connected to its network belongs to a predetermined group.