1. A control device for controlling a control target, the control device comprising a processor configured with a control program to perform operations comprising: operation as a command value generator configured to generate a command value: operation as a controller configured to control a control target based on the command value; operation as a predictor configured to predict a predictive output value of an output value to an input value with respect to a model of the control target and output the predictive output value; operation as a relational value calculator configured to calculate a prediction error as a relationship between the predictive output value of the predictor and a measured value of output of the control target; and operation as a relational value comparator configured to compare a first relational value to a second relational value, the first relational value being the prediction error in a reference state in which the control target is operated once to produce a prediction error reference characteristic, the second relational value being the prediction error in an operating state in which the control target is operated in a normal state.