| 摘要 |
Laser light is irradiated from a lateral side of an optical disk (2) set in place to a peripheral side face of the optical disk (2), return light from the peripheral side face irradiated with the laser light is detected, and a displacement of the optical disk (2) is measured based on the detected amount of return light. Also, the focus is adjusted in relation to the information recording surface correspondingly to the measured disk displacement. Thus, a displacement of an optical disk can be measured in an extremely simple manner and with an extremely high accuracy, and information can accurately be recorded or reproduced to or from the optical disk with the focus being controlled.
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