The invention implements an IP network system capable of causing a packet to reach a destination with reliability even if a trouble occurs to any of routers. The IP network system wherein packet communication is executed between respective hosts through the intermediary of a ring network comprised of at least a plurality of repeaters, the respective repeaters comprising an arithmetic and control unit for transferring a packet to the host at a transmission destination via a clockwise route, and a counterclockwise route, in the ring network, respectively, on the basis of transfer direction information contained in a tunneling packet generated on the basis of a packet transmitted from the host at a transmission source, as one of the respective hosts, to the host at the transmission destination, and subjected to multicast transmission to a multicast address where the host at the transmission destination belongs.