In shifting a flow that passes through a node onto a different alternative path which does not pass through the node passed through by the flow, a controller that controls a flow of each node under control determines alternative candidate paths, each of which has a same source and a same destination as a path associated with the flow to be shifted and does not pass through the node passed through by the flow to be shifted, determines the alternative path from the alternative candidate paths according to a predetermined criterion, and aggregates flows of each node on the alternative path.