A mobile node moves from a first IP (Internet Protocol) network to a
second IP network in a network system in which the first IP network
capable of executing communication in accordance with both first and
second kinds of IPs and the second IP network capable of executing
communication in accordance with only the first kind of IP are connected
with each other. When the mobile node communicates a message with other
nodes on the first network after its movement accordance with the second
kind of IP, a header for the movement containing both home and foreign
addresses of the first kind in IP is added to a header containing home
and foreign addresses in the second kind of IP, and put to the message,
is added. The message to which the movement header is thus added is used
for the communication between a first mobile agent on the first network
and a second mobile agent on the second network, or between the mobile
node and the first mobile agent.