| 摘要 |
PROBLEM TO BE SOLVED: To attain the accurate measurement of a stray light component included in the signal that is outputted from a light receiving part. SOLUTION: In steps S1-S5, an objective lens 22 is moved to the upper and lower limit positions of a focus moving range and the 1st and 2nd levels of an RF signal are stored when the lens 22 is set at its upper and lower limit positions respectively. The 1st and 2nd levels of the RF signal are compared with each other in a step S6. In steps S7 and S8, the levels of the RF signal set when the lens 22 is kept at its upper limit position, a focusing error signal and a tracking error signal are measured when the 1st level is higher than the 2nd one and then the levels of the RF signal set when the lens 22 is set at its lower limit position, the focusing error signal and the tracking error signal (stray light component) are measured when the 1st level is lower than the 2nd one. Then the offset of every signal is canceled according to the selectively measured levels in a step S9.
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