| 摘要 |
A disc discrimination circuit (4) detects the substrate thickness of a loaded optical disc (1), and supplies a detection output indicating that the loaded optical disc is a first optical disc with a large substrate thickness or a detection output indicating that the loaded optical disc is a second optical disc with a small substrate thickness, to an aperture ratio variable control unit (14). The aperture ratio variable control unit (14) controls an aperture ratio varying unit (16) in response to the detection output from the disc discrimination circuit (4), so as to vary the aperture ratio of an objective lens (7) to an aperture ratio suitable for the disc by shielding a part of a laser beam from the objective lens (7). Since the laser beam can be radiated at an aperture ratio corresponding to the substrate thickness of the optical disc, plural types of optical discs of different substrate thicknesses can be reproduced.
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