When an IPv4/v6 dual stack terminal communicating via TCP on IPv6 fails to promptly switch over to IPv4 after the ICMPv6 fails due to a soft error, the problem should be resolved at the prior stage server rather than on the complicated terminal. The server at a stage prior to the terminal receives TCP packets from the terminal to find the TCP connection status on the terminal. When the terminal connection status is SYN-SENT or SYN-RECEIVED, and the server receives an unaddressed soft error such as ICMPv6 “Destination Unreachable: no route to destination (ICMP type=1: code 0)” for that terminal, the server rewrites the ICMPv6 contents as a hard error such as “Destination Unreachable: communication with destination administratively prohibited (ICMP type=1: code 1)” and sends it the terminal.