In a capacitance type accelerometer used for an air bag system etc., a plurality of grooves crossing each other are provided on the surface of a movable electrode which is arranged to face a fixed electrode at a portion having a gap thereinbetween, and end portions of each groove open to an outside space at the portion between the movable electrode and the fixed electrode. The grooves can be provided on surface of the fixed electrodes. Ventilation between inside and outside of the facing portion is improved by opening of the grooves to outside space, and air damping which is caused at operation of the movable electrode at the gap between the electrodes is reduced, and, consequently, response speed of the movable electrode is increased.