PURPOSE: To prevent drop of the sensing accuracy of an acceleration sensor caused by the deadweight of a weight by forming it from a quadriangular plate of electrically insulative material, and furnishing movable electrodes, stationary electrodes, etc., on a fixing frame of an insulative substance which are located on the sides apart from one another.

CONSTITUTION: When a lateral acceleration is applied to a weight 5, a sideways movement takes place so that the distance between a movable electrode 51 and a stationary electrode 61 varies to cause a change in the electrostatic capacitance, and thus the acceleration is sensed. Even though the weight 5 moves down by its deadweight, the distances between the electrodes 51, 52 and 61, 62 do not change because of parallelism, and because the lengths in vertical direction of the electrodes 51, 52 and ones 61, 62 are good larger than the distance for which the weight 5 normally moves downward due to its deadweight, change in the mating areas of electrodes 51, 52 and ones 61, 62 is lesser. Accordingly there is no risk that the deadweight of the weight 5 deteriorates the sensing accuracy. If the vertical length of one of the electrodes is made smaller than that of the other, change in the mating area of these electrodes will further lessen.