1. A packet transfer apparatus comprising: a plurality of protocol processing units each connected to an IP (Internet Protocol) network; and a switch for switching IP packets among said protocol processing units, wherein each of protocol processing units connected to an IPv4 network on which IP packets each having an IPv4 address are transferred comprises: input-side address translating means for translating the address of an input packet received from the IPv4 network to an IPv6 address and inputting the address-translated packet to said switch; and output-side address translating means for translating, when an output packet having an IPv6 address is received from said switch, the address of the output packet to an IPv4 address and transferring the address-translated packet to said IPv4 network.