To perform invariably stable focus control.

A strain gauge 20 is fitted to a leaf spring of a focus actuator 7 and when a collision of an objective 8 against a disk 2 is detected at the time of focus rough adjustment, for example, the focus actuator 7 is elevated to make the objective lens 8; retreat, and a leaf spring displacement signal (s) corresponding to the strain that the leaf spring has in focus control is generated to move the focus actuator 7 between the upper displacement maximum value and lower displacement maximum value of the leaf spring according to the leaf spring displacement signal (s), a collision of the objective 8 against the disk 2 is detected according to the leaf spring displacement signal (s) from the strain gauge 20, and in the rough focus adjustment, a collision of the objective 8 against the disk 2 is detected to elevate, for example, the focus actuator 7, thereby making the objective 8 retreat.