PURPOSE: To prevent misjudging due to noise during judging the existence of abnormality by comparing an input signal with a threshold value.

CONSTITUTION: When an input signal X is within an upper limit A and a lower limit B, a normal counter C is counted up (step 4). When the count value becomes nc, the decision of normality is determined and malfunction judgment counters A and B are cleared (steps 5 to 8). Similarly, when the signal X is larger than the upper limit A and the counter A counts such states na times, malfunction A is determined and the counters B and C are cleared (steps 9 to 13).