1. A computer-implemented plant controlling device having an operation signal generator for generating an operation signal to be given to a plant, the plant controller comprising: a processor; a model configured to predict the value of a measured signal obtained when an operation signal is given to the plant; a database configured to store measured signal limit values set in advance; an external input interface configured to fetch measured signals from the plant; a measured signal database configured to store the values of the fetched measured signals; a plant model output value determining device configured to determine a plant model output value by using the measured signal obtained from the plant and at least one of the measured signal limit values set in advance, including a function configured to determine the plant model output value by using the measured signal limit values and at least one of an average value, a maximum value, and a minimum value that are calculated from the measured signals stored in the measured signal database; and a learning device configured to learn a method of generating a model input such that a model output, which is a prediction result yielded by the model, satisfies the plant model output value; wherein the operation signal generating device is configured to generate an operation signal with reference to a database that stores results of learning obtained by the learning device; wherein the learning device configured to learn so as to attain the determined plant model output value.