PURPOSE: To obtain an apparatus which enables the decision on abnormality only when a sensor gets truly defective, by a method wherein abnormality of the sensor is detected with a pulse cycle being above a set value and an abnormality signal is outputted when changes in the revolutions of an engine exceed a specified value within a specified time afterwards.

CONSTITUTION: An air flow sensor 3 is set in an intake path 2 of an engine 1. The sensor detects the number of Karman vortexes U generated per unit time as caused in the path 2, by an ultrasonic modulation to measure intake air flow while outputting a pulse signal corresponding to the measured value. The cycle of the pulse is counted with a time counter when either the voltage of a battery 8 or the revolutions of the engine detected with a revolutions detection sensor 5 exceeds, a condition meeting an abnormality decision requirement. With the counts exceeding an abnormality decision value temporarily, a CPU 13 makes a virtual abnormality decision. Thereafter, if the revolutions of the engine change beyond a specified rate of change within a specified time, an alarm signal from a control circuit 10 activates an alarm 6, which informs of abnormality of the sensor 3 by light, heat or the like.