Hall elements detects magnetic signals of a rotational member which is secured to the crank shaft or the cam shaft of an engine and converts them to an alternating electric signal. The electric signal is amplified by a differential amplifier and applied to a peak-detecting-type wave-shaping circuit which generates a rotational position signal and a control signal. A reset signal generator generates an interrupting signal in response to the control signal and is applied to a NAND circuit which is connected between the wave shaping circuit and an output circuit. When a power switch is closed, the power source voltage sometimes drops to an extremely low value for providing the rotational position detecting signal. At this moment, the reset signal generator generates the interrupting signal and closes the NAND circuit so that erroneous signal of the rotational position detecting signal is prevented.