1. A plant controlling device having an operation signal generator for generating an operation signal to be given to a plant, which is a control target, the plant controller comprising: a model for predicting the value of a measured signal obtained when an operation signal is given to the control target; a target model output value determining device for determining a target model output value by using the measured signal obtained from the control target and a measured signal limit value set in advance; and a learning device for learning a method of generating a model input so that a model output, which is a prediction result yielded by the model, satisfies the target model output value; wherein the operation signal generating device generates an operation signal with reference to a database that stores results of learning obtained by the learning device.