1. A control device for controlling a control target, the control device comprising: a predictor configured to output a predictive value of an output value to an input value with respect to a model of the control target; a relational value calculator configured to calculate a relational value indicating a relationship between the predictive value and a measured value of output of the control target; and a relational value comparator configured to compare a first relational value to a second relational value, the first relational value being the relational value in a reference state in which the control target operates in a specific state, the second relational value being the relational value in an operating state in which the control target is operated.