A heat-ray type of airflow meter has a signal processor for converting a detected intake air quantity into a frequency signal. An engine controller has a conversion table for converting the frequency signal into an air quantity. The signal processor and the conversion table have features such that the frequency increases as the magnitude of a positive air quantity increases, and the frequency decreases as an absolute value of a negative air quantity increases. In the conversion table, a prescribed positive air quantity value is assigned a dummy output for frequencies lower than a prescribed threshold value. Under normal circumstances, frequencies lower than a minimum value are not used. The frequency decreases to near 0 Hz when there is a disconnection or a short circuit, the dummy output is therefore output, and an injection quantity equal to or greater than a misfire limit is ensured.