A transmission destination of a packet to be transmitted by a web server (11) and an application server (21) incorporated in the same machine (30) is set to a loopback device (50). When a packet is received from one of the servers (11 and 21), the loopback device (50) decides whether or not a combination of a source IP address, a source port number, and a destination port number in the packet is known. If the combination is known, the source IP address and a destination IP address of the received packet are exchanged, and the packet is transmitted to the other server. Further, the packet received from the one of the servers and the packet transmitted to the one of the servers are copied, and the copied packets are transmitted to a communication state visualizing machine (90).