Abstract: An axially focusable objective lens is retained in a neutral position in the axial focusing direction by means of at leaast one magnetic member disposed inside or near the ends of magnetic gaps formed by yokes and magnets. The magnetic member is mounted on the periphery of a retaining tube which carries the objective lens and is positioned in the magnetic gaps which exhibit a non-uniform magnetic field distribution so that the magnetic member is subjected to a force acting in a direction in which the potential energy becomes stable. This force is utilized as a magnetic spring in the axial focusing direction. The magnetic spring requires no additional space thereby enabling a reduction in the size of the actuator and achieving excellent temperature characteristics.