PURPOSE: To discriminate the abnormality of a sensor by recognizing the continuous abnormal condition and the repeated abnormal condition such as contact failure of a connector portion or the like.

CONSTITUTION: In the step 102, the abnormality of an input signal is discriminated according to a signal from a sensor, and according to the abnormality discrimination, the first counter is incremented in the step 102. When the normality is discriminated, the first counter is cleared in the step 107. In the first counter, the condition where abnormality is continuously caused is counted and compared with a preset threshold value (b) to discriminate the continuous abnormality. Every time the input signal is changed from the normality to the abnormality, the second counter is incremented in the step 104, the frequency of occurrence of abnormality is integrated and counted. When it is judged that the count value exceeds a threshold value (a) in the step 105, instantaneous abnormality such as contact failure of a connector portion or the like is decided.