A node device and a label switched path loop detection method which are capable of detecting a label switched path loop efficiently and quickly are disclosed. In a node device for carrying out the label switching, an ingress node information is included in a message that is to be successively transmitted from the upstream side in order to set up a label switched path, and the fact that the message with the same ingress node information for the same packet flow will be received from a previous hop node that is different from before if the label switched path has a loop formed thereon is utilized. Also, at a time of transmitting a message for the purpose of setting up a label switched path, if a node at which the label switched path for that packet flow already exists on the upstream or downstream side is encountered, this already existing label switched path is traced along the same or opposite direction as the flow, and the label switched path is set up if no loop is detected after tracing to the end.