| 摘要 |
An object of the invention is to suppress an influence of a wavefront aberration which is generated at the time of correcting a third-order spherical aberration, and realize satisfactory information recording and/or reproducing. A collimator lens 4 corrects a third-order spherical aberration which is generated depending on the thickness of a light transmissive layer from a surface of a first optical information recording medium 30, corresponding to an optimal substrate thickness of a first objective lens 6, to an intended information recording surface, assuming that the light transmissive layer thickness of the first optical information recording medium 30 which minimizes a residual third-order spherical aberration at the time of incidence of parallel light into the first objective lens 6 is defined as the optimal substrate thickness of the first objective lens 6. The optimal substrate thickness of the first objective lens 6 is defined in such a manner that a variation of a wavefront aberration which is generated on the information recording surface of the first optical information recording medium 30 farthest from the first objective lens 6 at the time of correcting the third-order spherical aberration, and a variation of a wavefront aberration which is generated on the information recording surface of the first optical information recording medium 30 closest to the first objective lens 6 at the time of correcting the third-order spherical aberration are set equal to each other.
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