Abstract: A process control system 100 includes a plurality of controller devices 10 each of which performs process control on a plant, and an input-output device 20 that is connected to a target device of the process control. The input-output device 20 is installed in a different on-premise environment from the plurality of controller devices 10. Each of the controller devices 10 is connected to the input-output device 20 by a different closed network 40, and transmits and receives information on the process control on the plant to and from the input-output device 20.

Abstract: A controller is a controller forming, with another controller, a redundant controller. The controller includes a communication function unit that performs communication with an external device capable of executing control calculation needed for process control for a plant, and a redundancy management unit that switches states of the controller between an active state where a result of the control calculation obtained from the external device via the communication function unit is reflected in the process control and a standby state where the result of the control calculation is not reflected in the process control. The redundancy management unit switches the states of the controller to bring one of the controller and the another controller into the active state.