PURPOSE: To prevent the direction of acceleration from being misdetected owing to variation in source voltage by making a polarity decision without converting the level of a detection signal and separating the sign of the detection signal, and then performing A/D conversion.

CONSTITUTION: A sign detecting circuit 20 decides the polarity of a detecting circuit and generates and supplies a one-bit sign signal to a microprocessor 22. An absolute value signal from an absolute value circuit 21 is digitized by an A/D converting circuit 15 and supplied to the microprocessor 22. The microprocessor 22 processes the digital detection signal consisting of said one-bit digital sign signal and plural-bit digital absolute value signal digitally to perform suspension control, acceleration display control, etc. Thus, the signal is supplied to the sign detecting circuit 20 directly without being shifted in level to decide the polarity, so even if the source voltage varies, the polarity of acceleration is never misdetected.