This disclosure describes, for a example, a plant control system comprising: program editing means for creating a control program including an interlock condition and diagnosis of interlock; plant control means for executing a program object in the control program; and interlock diagnosing means for updating and displaying satisfaction/dissatisfaction of the interlock condition at a predetermined point before the present, further specifying only a source factor variable of the dissatisfaction even if plural stages of interlock circuits are formed, and automatically displaying the source factor variable in a graphic language, the interlock diagnosing means having a logical operation unit and a display unit.