PURPOSE: To simplify the constitution and to detect acceleration in two axial directions by providing an acceleration detecting element with at least four electrodes for the detection of the quantity of electricity in four directions.

CONSTITUTION: When a vehicle is accelerated or decelerated suddenly, acceleration is generated in the front-rear direction of the vehicle body and the acceleration detecting element 13 defects forward or backward. Consequently, one of divided pieces 13A and 13C is compressed and the other is expanded. For example, the divided piece 13A is compressed and then generates an electromotive force. This electromotive force is detected through electrodes 22 and 23 and a controller decides the acceleration of the vehicle from the quantity of this electromotive force. Further, acceleration is generated in the right-left direction of the vehicle at the time of changing courses or turning, and the element 13 deflects to the right or left. Consequently, one of the divided pieces 13B and 13D is compressed and the other is expanded; and, for example, the divided piece 13B is compressed and then generates an electromotive force. This electromotive force is detected through electrodes 23 and 24 and the controller decides the acceleration of the vehicle from the quantity of the electromotive force.