A field control system comprising: a plurality of field devices; a plurality of field controllers; and an operation monitor, wherein each of the plurality of field controllers is connected to a control network and is configured to execute a computation processing for controlling at least one of the plurality of field devices according to a given control cycle while executing a data communication between said field controller and said field device, said each field controller comprising: a communication unit configured to execute the data communication with said field device; and a control computation unit configured to execute the computation processing independently from the communication unit; the operation monitor is connected to the control network and configured to operate and monitor the field device, the operation monitor comprising: a network clock which provides a common network time to the control network, wherein the control computation unit and the communication unit of each field controller are configured to repeatedly execute the computation processing and the data communication in synchronism with each other in accordance with timing according to a certain cycle of a timer clock based on the network time of the network clock, wherein the plurality of field controllers are adapted to perform communication through the control network with one another. a plurality of field devices; a plurality of field controllers; and an operation monitor, wherein each of the plurality of field controllers is connected to a control network and is configured to execute a computation processing for controlling at least one of the plurality of field devices according to a given control cycle while executing a data communication between said field controller and said field device, said each field controller comprising: a communication unit configured to execute the data communication with said field device; and a control computation unit configured to execute the computation processing independently from the communication unit; the operation monitor is connected to the control network and configured to operate and monitor the field device, the operation monitor comprising: a network clock which provides a common network time to the control network, wherein the control computation unit and the communication unit of each field controller are configured to repeatedly execute the computation processing and the data communication in synchronism with each other in accordance with timing according to a certain cycle of a timer clock based on the network time of the network clock, wherein the plurality of field controllers are adapted to perform communication through the control network with one another. a communication unit configured to execute the data communication with said field device; and a control computation unit configured to execute the computation processing independently from the communication unit; the operation monitor is connected to the control network and configured to operate and monitor the field device, the operation monitor comprising: a network clock which provides a common network time to the control network, wherein the control computation unit and the communication unit of each field controller are configured to repeatedly execute the computation processing and the data communication in synchronism with each other in accordance with timing according to a certain cycle of a timer clock based on the network time of the network clock, wherein the plurality of field controllers are adapted to perform communication through the control network with one another. a network clock which provides a common network time to the control network, wherein the control computation unit and the communication unit of each field controller are configured to repeatedly execute the computation processing and the data communication in synchronism with each other in accordance with timing according to a certain cycle of a timer clock based on the network time of the network clock, wherein the plurality of field controllers are adapted to perform communication through the control network with one another.