1. A routing control method in a mixed environment of a hierarchial network and a non-hierarchial network, comprising: assigning the non-hierarchial network a virtual hierarchy number that corresponds to a hierarchy number in the hierarchial network, attaching the virtual hierarchy number to a packet to be relayed at a router located at an entrance from the non-hierarchial network to the hierarchial network when the packet is to be relayed between non-hierarchial networks via the hierarchial network, performing a hierarchial routing control by the virtual hierarchy number within the hierarchial network, and removing the virtual hierarchy number from the packet to be relayed at a router located at an exit from the hierarchial network to the non-hierarchial network.