A plant control apparatus includes a plant model and a control input determining device. The plant model estimates a control variable of a plant based on an input including a control parameter to adjust a first control variable of the plant. The control input determining device is configured to correct a temporary value of the control parameter so that a first control variable model value satisfies a predetermined constraint. The control input determining device is configured to determine a control input for the plant based on the temporary value of the control parameter that has been corrected. The first control variable model value is defined as an estimated value of the first control variable calculated by the plant model when the temporary value of the control parameter is input to the plant model.