Acceleration sensor

The present invention relates to an acceleration sensor, and more particularly, relates to an acceleration sensor attached to an inclining member such as a seatback or the like of a vehicle, which sensor is not affected by inclination of this inclining member in detecting acceleration of the vehicle. An object of the present invention is to obtain an acceleration sensor that can reliably detect acceleration when inclination of an inclining member is within a fixed range, and that can reduce unnecessary movement of relevant members when inclination of the inclining member is outside of the range, so that space efficiency in an inner portion is high. In the present invention, a gear of a fixed gear fixed at a seat cushion is formed in only a portion in the circumferential direction, and a portion at which the gear is not formed is a sliding surface. One of teeth among a plurality of teeth of a pinion of a revolving gear is short, to form a notched tooth portion. In a state in which the forward-collapsed angle of a seatback exceeds a predetermined angle, the notched tooth portion faces the sliding surface and the revolving gear body no longer rotates. At this time, since a bracket at which a sensor ball is mounted also does not rotate, the bracket is held at a fixed position relative to a frame plate. Accordingly, a space formed in frame plates in consideration of rotation of the bracket does not need to be large, and space efficiency is high.