Provided is a multiplex transmission system capable of preventing occurrence of a mistake on connection of a client device to a multiplex transmission device. The multiplex transmission system includes: an extraction means configured to extract identification information from a signal inputted to a client port of each of a first multiplex transmission device 100 and a second multiplex transmission device 200; and a switch control unit 340 configured to control a switch unit 330 to connect the client port of the first multiplex transmission device 100 with the client port of the second multiplex transmission device 200 on the basis of a result of collation between identification information extracted from a signal inputted to the client port of the first multiplex transmission device 100 and identification information extracted from a signal inputted to the client port of the second multiplex transmission device 200.