Abstract: In an automatic control system, controls receive an input indicative of a deviation between feedback and demand signals and produce an output which in turn is applied to a plant to control it. A diagnostic device includes a model having an equivalent characteristic to the controls and receives the deviation signal. When a noise coming into the automatic control system is detected or when a deviation between the outputs of the model and controls is sufficiently small, no model operation is performed, and the output of the controls is determined as that of the model. However, when the deviation between the outputs of the model and the controls is large, the model is supplied with an input thereby performing a model operation. A resultant output of the model is then compared with the output of the controls to produce an alarm indicative of an abnormal state of the controls in accordance with a deviation between these outputs.