A system according to the invention has a plurality of sensors, a system control circuit and a plurality of actuators. State data, supplied from the sensors, is compared with characteristic data in the system control circuit. From this comparison, it is judged whether a fault has occurred depending upon whether the state data is relatively lower or higher. If a fault has occurred, fault inference fault is made on the basis of a comparison between qualitative data and fault diagnosis knowledge which are stored in the system control circuit. Then a judgement is made as to which of the elements in an image forming apparatus is abnormal. An actuator operating signal thereafter is generated as a result of this judgement.