PURPOSE: To improve driving efficiency by changing the pitch of a spiral cut line to increase dimensions of the section in the axial direction in the center part in the axial direction and to reduce those toward end parts in the axial direction.

CONSTITUTION: Since a feeding pitch P for spiral machining of a cylindrical conductive material 17 is gradually coarsened from both ends toward the center of the conductive material 17, the number of winding of a driving coil 11a per a unit length is gradually increased from the center toward both ends. When the current to the coil 11a is changed, the coil 11a is finely moved in the vertical direction to drive an objective lens 8. In this case, the distribution of the magnetic flux density of the magnetic field formed in a magnetic gap 14 between a flange 11e of a yoke 11c and a pole piece 11d is effectively used to perform linear fine adjustment. That is, an electromagnetic force is effectively generated even in the part, which is distant from the magnetic gap 14 having a lower magnetic flux density, by larger numbers of winding of the coil 11a in both ends. Thus, the driving efficiency is improved.