1. A packet transfer apparatus comprising: a plurality of protocol processing units, each of which is connected to an IP (Internet Protocol) network and provided with a function of converting each of IP packets received from the IP network into an internal packet having an internal header with internal routing information and extracting an IP packet from each of internal packets transferred from the other protocol processing units to output the IP packet to the IP network; and a switch for switching internal packets including IPv4 packets, each having an IPv4 address, and internal packets including IPv6 packets, each having an IPv6 address, among said protocol processing units based on the internal routing information of each internal packet, wherein each of said protocol processing units connected to an IPv4 network comprises: input-side address translating means for translating the IPv4 address of an input packet received from the IPv4 network to an IPv6 address so that the address-translated IPv6 packet is supplied to said switch after being converted into an internal packet; means for storing operation mode designation information indicating whether execution of address translation by said input-side address translating means is necessary or not; packet transfer control means for transferring input packets received from said IPv4 network either in an IPv6 routing mode in which each of the input packets is supplied to said switch after translating the IP address of the input packet to an IPv6 address or in an IPv4 routing mode in which each of the input packets is supplied to said switch without performing address translation, in accordance with said operation mode designation information; output-side address translating means for translating, when an internal packet including an output packet having an IPv6 address is received from said switch, the IPv6 address of the output packet extracted from the internal packet to an IPv4 address so that the address-translated IPv4 packet is transferred to said IPv4 network; and control means having a memory for indicating a network address version of the IP network connected to the protocol processing unit, wherein said control means determines the address version of the output packet extracted from the internal packet received from said switch by referring to an IP header of the output packet and transfers the output packet to said IPv4 network without going through said output-side address translating means when the output packet has an IP address version matched with the network address version indicated by said memory.