| 摘要 |
In an optical disc device of the present invention, time differences among the focus error signal corresponding to the surface of the optical disc, the focus error signal corresponding to the first reflection layer, and the focus error signal corresponding to the second reflection layer, all of which are obtained through the light receiving element when a relative distance between the objective lens and the optical disc is linearly changed in a focus search of the optical disc are measured. The thickness of the cover layer from the surface of the optical disc to the first reflection layer and the thickness of a cover layer from the surface of the optical disc to the second reflection layer are detected on the basis of these time differences.
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