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.