To provide a monitoring system wherein a communication load does not increase even if the number of monitors increases, capable of performing monitoring without increasing a PLC load.

One of the plurality of monitors 10, 20, 30, e.g. the monitor (the main monitor) 10 provided from the beginning periodically performs requirement 1, 2 of data to a PLC 40 by a polling method, receives response data 1', 2' from the PLC 40, converts them into engineering values, and displays them. The extended other monitors (the sub-monitors) 20, 30 capture all pieces of data of network information, take in only transmission data (the response data) 1', 2' of the PLC 40, and perform processing. Thereby, only the data requirement for the display only from the monitor 10 is needed, the data requirement for the display from the monitors 20, 30 is not needed to reduce the loads of the PLC 40 and a network 100.

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