Control system in which communication between devices is controlled based on execution condition being satisfied, gateway device used in the control system, and control method for the control system

In a control monitoring system in which an information system is connected with a control system, the control system includes a control device that performs at least one of control and monitoring of a control target, a computer that performs a process related to the control or the monitoring, and a gateway device that relays a communication message between the control device and the computer. The gateway device acquires a predetermined state index related to a first communication message when the first communication message is received from the computer, determines whether or not a predetermined execution condition is satisfied based on the state index, transmits a second communication message based on the received first communication message to the control device when the execution condition is satisfied, and does not transmit the second communication message to the control device when the execution condition is not satisfied.