1. A plant control system having a measurement signal data database in which to save previous measurement signal data, a model to estimate a value of measurement signal data used at a time when an operation signal is given to a plant, an operation signal learning unit to learn a method of generating a model input, which is equivalent to an operation signal, so that a model output, which is equivalent to the measurement signal data, attains a target value in the model, an evaluation function calculating unit to calculate an evaluation function value, which indicates an attainment degree of the target value, from the model output obtained as a result of an operation carried out by the operation signal learning unit for the model, and a learning information database in which to save information about records of evaluation function values obtained by the operation signal learning unit and learning and information about model output learning records, the plant control system using the measurement signal data obtained from the plant to calculate the operation signal for operating the plant and sends the calculated operation signal to the plant, wherein the control system comprises an evaluation function adjusting unit to receive setting condition information used for evaluation function adjustment from the learning information database, adjusting at least two evaluation function parameters used for calculation of an evaluation function in the evaluation function calculating unit, and saving information about an adjustment result including an evaluation function parameter adjustment record, wherein the at least two evaluation function parameters include: at least one of a parameter which determines the expected value of the evaluation function value to change within a predetermined range of the model output or a reference model output, which is a weighted sum of the plurality of model outputs, and a parameter used as a threshold value to obtain an optimum value of the evaluation function; and the evaluation function calculating unit has a feature to calculate the evaluation function value from the model output or the reference model output obtained as a result of an operation performed by the operation signal learning unit for the model.