A mobile IP communication scheme for supporting a mobile computer moving over different address spaces is disclosed. A packet relay device for relaying packets having an address of the mobile computer device as a destination or source is provided at a border between a private address space and a global address space, where the packet relay device has a packet receiving unit for receiving a packet in a first format using a global address which is transmitted by the mobile computer for a location registration from a visited site managed by a global address system, and checking a content of the packet, and a packet transfer unit for transferring the packet in a second format using a private address, to a correspondent computer in a home network of the mobile computer managed by a private address system, according to the content of the packet.