An actuator includes a shear-type piezoelectric element of predetermined thickness and an opposed electrode formed on two electrodes in spaced relationship on a base member. The opposed electrode is displaced in accordance with the direction of polarization of the shear-type piezoelectric element upon applying a voltage between the two electrodes. The opposed electrode thus constitutes an operating section of the actuator. The actuator can be incorporated as a part of the head actuator to constitute a head-positioning mechanism capable of displacing the head by a minuscule distance independently of the operation of the head actuator.