The block #1 capsules a user IP layer by a UDP/IPv6, as well as detecting a destination IP address and specifying a QoS class from a DSCP embedded in a TOS field of an IP header of the user IP layer to transfer data to the block #2 through a UDP port correlated to a corresponding link. The blocks #2 and #3 execute processing of protocols divided into functional blocks with respect to a payload part of a UDP packet obtained from each UDP port to transfer the data to its subsequent block through a UDP port correlated to a corresponding link. The block #4 embeds, into a TOS field of an IP header of an IP packet obtained from each link, a DSCP value of a QoS class correlated to the link.