PURPOSE: To improve the accuracy of fault detection, to reduce required time and to improve reliability by deciding a second fault state for which abnormality detection is detected in the prescribed number of times of trips.

CONSTITUTION: Whether or not an engine cooling water temperature transmitted from a water temperature 22 through an input interface circuit 13 and an A/D converter 14 is within a prescribed range is decided first. When it is decided that the water temperature is not within a normal range, whether or not the value of a TIMER exceeds one second is decided. Then, when it exceeds one second, it is defined that an abnormal state continues for sufficient time, the counts in a counter CNT1 and a cumulative counter CNT2 are increased and also a flag CNTFLG is turned ON. Then, when the abnormality is detected continuously while the value of the CNT1 is more than two, that is for more than two trips, storage is performed as 'a complete fault'. Also, when the abnormality is detected while the value of the CNT2 is more than 10, that is, when the abnormality is detected in the trips of cumulative 10 times even when the abnormality is not detected in the consecutive trips, the storage is performed as 'an incomplete fault'.