In a system of controlling the operation of a plant by using a digital computer, operation control procedures are classified according to various states of the plant process variables. The control procedures comprise essential break points of the plant running, and process controlling operations, process supervision operations, information display operations and other processings related to the Tables corresponding to the operation control procedures are prepared containing classified items which are described according to a predetermined format. The contents of the tables are converted into machine words and stored in the memory device of the computer. The computer is provided with means responsive to a changed process signal from the plant for selecting a table corresponding to the changed process signal (representing a changed state) to form an operating signal which is used to control the plant. There is also provided means for displaying and modifying the content of a selected table. This permits ready modification or addition to the tables by the computer itself.