PROBLEM TO BE SOLVED: To help easily grasp whether the output signal of a main controller in an operation state is normal or not by comparing the relation between a main controller output signal value and an operation process value with operation result data. SOLUTION: Reactor output, a water-supply discharge header pressure, and a reactor pressure are inputted to an extraction means 8, and the past operation cycle result data of a plant state corresponding to it are extracted from a database 6. Data are created so that a result region can be displayed based on a group of the extracted result data. In this case, by displaying a rectangular region, the result region is shown and is compared with a plotted point according to a process value on current operation, thus fully putting together information regarding a plant control system and information regarding the plant with improved balance and supporting the monitoring of the validity or the like of a main controller output signal value for the current operation state.