A plant control device includes a feedback controller (54) that determines a control input of a plant (56), and a reference governor (52) that corrects an initial target value and outputs the corrected initial target value to the feedback controller (54). The reference governor (52) determines a target value satisfying an upper limit constraint to be a target value candidate that minimizes a value of an evaluation function. The reference governor (52) determines the target value satisfying the upper limit constraint to be a value of the upper limit constraint when a value acquired by substituting, with the value of the upper limit constraint, a variable of a differential function acquired by differentiating the evaluation function with respect to the target value candidate is greater than or equal to zero.