To prevent the wrong decision to a transient signal by deciding the failure of a control means based on the deviation between a signal level at which the detection signals preceding up to a prescribed generation are stored and the level of a signal that is received via a transmission means.

The CPU 7 and 8 store the detection signals preceding up to a prescribed generation and transmit the signals obtained by applying the arithmetic processing of the same contents to the signals detected by the sensors 3 and 4 to each other via the signal lines 14 and 15. Then each of both CPU 7 and 8 compares its on detection signal with the signal received from the other CPU to decide a failure. In other words, a PWM signal serving as the output of the CPU 7 is inputted to the CPU 8 via an input interface 11. The cycle of the CPU 7 is compared with its duty ratio based on the cycle detected from the PWM signal, the duty ratio and the signal that is received via the line 14. Thus, the failure of a control means is decided.