An acceleration sensor, for detecting the acceleration of a motion made by a head actuator that includes a carriage which is borne on a base of a disk drive so that it can pivot and a head suspension fixed to the distal part of the carriage, is formed using at least one shearing piezoelectric element having an electrode formed on both sides thereof. One of the electrodes is attached to the distal part of the carriage with a suspension base between them, while the other electrode is attached to the proximal part of the head suspension with a head mounting block between them. The shearing piezoelectric element is realized with one shearing piezoelectric element that is polarized in a direction orthogonal to the longitudinal directions of the carriage. Consequently, a large output is produced with only a small weight added to the carriage of the disk drive.