PURPOSE: To detect completely the abnormal output of a specific port by integrating the output of the specific port of a microprocessor MPU with no saturation and applying this output to a hysteresis comparator.

CONSTITUTION: The output A of an I/O port of a MPU is integrated with no saturation by an integration circuit R'C' having a large time constant. Then this output signal B of integration is supplied to a hysteresis comparator CMP1'. The CMP1' sets upper and lower levels of reference voltage ref1/ref2 within the amplitude of the output B when the signal A has a normal output. The amplitude of the integration output B or the average level is different from those normal values in case the output signal A of the I/O port produces the output having different frequency or duty from those of a normal state. Therefore, the CMP1' stops the output variation when the signal B gets out of the range ref1/ref2 set by the CMP1'. A program runaway is decided if said stop state of the output variation lasts for a fixed period of time. Thus the MPU is reset by a CMP2.