Abstract: A delay time of each node is acquired at a source node, and whether the acquired delay time is normal or abnormal is automatically recognized, thereby enabling an abnormal node to be detected in the source node. A test packet is transmitted from the source node to the destination node through the respective relay nodes, a relay delay time of each relay node, which has been measured in the each relay node, is added to the test packet, the source node is designated as a destination of the test packet received in the destination node to return a test response packet, the relay delay time of the each node is extracted from the test response packet received in the source node that has received the test response packet, and an abnormal node is detected by an abnormal state detector disposed in the source node according to the extracted relay delay time of the each node.