PURPOSE: To securely detect a runaway state by a simple circuit by comparing a delay time generated on the circuit side by utilizing an external varying signal with the delay time at a microprocessor.

CONSTITUTION: An engine speed signal 12 is used as the external input signal and made into a pulse train, which is inputted to a microprocessor 1; and the microprocessor 11 delays an input signal obtained through a signal input terminal 11c by a specific time and outputs it from a signal output terminal 11b. A delay circuit 13, on the other hand, delays the engine speed signal 12 by the same delay time with the microprocessor 11. The signals of those two systems have waveforms which vary in signal voltage at the same timing and those signals are voltage-compared by a comparing circuit 14 with each other to monitor the function of the microprocessor. If there is a voltage difference between the signals of those two systems, the microprocessor 11 is reset through a reset terminal 11a. Namely, if the microprocessor 11 enters a runaway state, the microprocessor is reset forcibly to stop running away, and its function is recovered.