PROBLEM TO BE SOLVED: To reduce the possibility of inhibition of normal operation of a plant caused by the state where a signal outputted from an object to be controlled is not normally transmitted. SOLUTION: The transmission monitoring system for monitoring a control target transmission signal outputted together with a control target identification code from a control target such as a valve 44 is provided with a power cable 10, a plurality of transmission devices 20 and a central control unit 50 as a monitoring apparatus. The transmission devices 20 each superimpose a transmission device transmission signal obtained by adding the transmission device identification signal to the control target transmission signal when its own device is normally operated and to an abnormal signal when an abnormality occurs in its own device so that the obtained signal is superimposed on the power cable 10, and transmits the obtained signal. When the transmission device transmission signal includes the control target identification signal, the central control unit 50 stores the transmission device transmission signal in association with the transmission device identification code, and when the transmission device transmission signal includes an abnormal signal, the central control unit 50 judges that signal exchange is impossible with the control target identified by the control target identification code stored in association with the transmission device identification code. COPYRIGHT: (C)2010,JPO&INPIT
