When the position of the pickup 14 is normalized, a lead screw 29 is
rotated by a stepping motor to move the pickup 14 to the outermost
circumferential position, whereby a preset counter is preset to the
maximum value CTout corresponding to the outermost circumferential
position. The counter counts proportionally to the number of driving
pulses to be supplied to the stepping motor, the lead screw 29 rotates
till the counting value of the counter gets "0", so that the pickup 14
moves near the outermost circumferential track of the optical disk in the
lead-in area to make the set up. The shape of the tooth flank of the lead
screw 29 and the inclination of the engaging projections 27, 28 are
designed such that the step out is caused between the engaging projections
27, 28 and the lead screw 29, when the pickup 14 is moved to the outermost
circumferential position. Even if this step out is caused, the engaging
projections 27, 28 only engage strongly with the lead screw 29.
