A transmitter router search apparatus 26 transmits an inspection IP packet to an inspection target network connection apparatus via an intranet 10. The source and destination IP addresses of the inspection IP packet are designated respectively as the IP addresses of a recipient router search apparatus 28 and the inspection target network connection apparatus. A TCP packet, including predetermined ID information, is included in the data portion. The inspection target network connection apparatus generates an IP packet corresponding to the received inspection IP packet. Since the transmission source and destination of the response IP packet are respectively designated the transmission destination and source of the inspection IP packet, the recipient router search apparatus 28 receives the response IP packet via the Internet 19, so long as the inspection target network connection apparatus is a router.