The plant control system has a measurement signal data database, a model to estimate the value of measurement signal data used at a time when an operation signal is given to the 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. The plant control system also has an evaluation function calculating unit to calculate an evaluation function value from the model output obtained as a result of an operation carried out by the operation signal learning unit for the model, and an evaluation function adjusting unit to adjust evaluation function parameters used in calculation of an evaluation function.