A fault-detecting apparatus comprises a model simulating the input-output characteristics of an object for fault detection. The model includes statics and dynamics and is impressed with the same input as the object for fault detection. The output of the model is compared with the output of the object for fault detection, and if the deviation is great, it is indicated that a fault has generated in the object for fault detection. In order to determine whether or not correction of the model is possible, the degree of the deviation is determined from an average value of the time-series signals produced from various parts of the fault-detecting apparatus during a certain period of time. For this purpose, a variation is calculated, for example; and if it is smaller than a predetermined value V.sub.L, a normal condition is judged and the statics correction is considered possible, while if the variation is larger than V.sub.H (which in turn is larger than V.sub.L), a transient condition is judged, and the dynamics correction is considered possible. Correction of the characteristics is effected by processing the data applied to the fault-detecting apparatus for the previous certain period of time.