PURPOSE: To make it possible to perform self-diagnosis which is not affected by environmental conditions such as temperature by operating the difference between the output values of a plurality of sensors, and generating a fault signal when the result is deviated from the range of the specified reference values.

CONSTITUTION: When the signal of no shaft is received from a means 19 for confirming the presence or absence of a cam shaft in a judging part 10, the difference between the electric signals which are the results of the computations of an operating part 9 is sent into a comparing part 16, and the self-diagnosis is performed. In the self- diagnosis, the inputted electric signal is compared with the reference value which is set with a reference-value setting part 13 in a comparing part 12 in a machine-type discriminating part 22. When the signal is within the range of the reference value, the normal signal is sent into a discriminating part 18 in a self-diagnosis part 15. When there is the signal which is deviated to the outside of the range of the reference value, the fualt signal is sent into the discriminating part 18, and the abnormality is notified with, e.g. a buzzer and the like. In this way, even if the output values as a whole are increased or decreased, the self-diagnosis of the sensor 1 can be accurately performed.