An actuator for a magnetic disk device causes movement of a magnetic head in the radial direction of a rotating magnetic disk. Although this magnetic head is floated above the magnetic disk by a stream of air induced by the rotation of the magnetic disk at a high speed, an excessively high velocity of the air stream causes excessive vibrations of a soft gimbal resiliently supporting the magnetic head. In the actuator of the present invention, a head arm supporting the magnetic head through the gimbal is formed with a wind-breaking projection preventing direct impingement of the air stream against the gimbal and magnetic head, thereby minimizing vibrations of the gimbal and magnetic head.