A packet switching is performed according to a routing information for
relaying particular packets which is produced by the route computation
based on the information of a received packet and the kept
parts-of-routing-information useful as a basis for the route computation.
At this time, the route through which the packet is relayed is determined
on the basis of the kept parts-of-routing-information before the route
computation. Then, the received packet is relayed according to the
determined route. Next, a new routing information is produced by the
route computation based on the information of the received packet and the
parts-of-routing-information. Thereafter, the succeeding packets of the
same route are relayed on the basis of the new routing information.