1. A plant control system comprising: an object to be controlled and having a control device for effecting control; a detector for detecting a process variable of said object to be controlled; a setting device for setting a target value of said process variable; and an adjuster for controlling said control device of said object in accordance with a difference between an output of said setting device and an output of said detector, wherein said plant control system further comprises storage means for storing a limit value established for said process variable for a purpose of protecting said object to be controlled, difference determining means for determining a difference between said limit value and a detected said process variable, and means for correcting a characteristic of said adjuster in accordance with an output of said difference determining means.