A detour path determination method wherein each node on a determined detour path stores in advance first and second nodes upstream of each node as well as an identifier(s) of a packet directional path(s); a number of an identifier(s) of a packet directional path(s) required for switching in each link extending from a downstream node one node downstream of each node on the detour path are set for respective nodes on the detour path beginning from the terminal node upstream; the identifier(s) of the packet directional path(s) set as above, and an identifier(s) of a corresponding packet directional path(s) at a start node of the detour path, are sent to a node one node upstream of the above each node; the start node updates a identification data table at the node, by an identifier(s) of a packet directional path(s) set downstream of the start node based on the corresponding identifier(s) of packet directional path(s) at a start node of the detour path, when the start node receives the above identifier(s) (FIG. 1).