THREE-BEAM OPTICAL HEAD

PURPOSE:To execute the exact tracking without generating vignetting of a sub-beam by making a size of a grating part of a diffraction grating smaller than the luminous flux diameter of an incident beam to the diffraction grating. CONSTITUTION:As for a diffraction grating 53, a size of a grating part 53a is set so as to become smaller than a shape of an incident beam to the diffraction grating 53. That is, by making the luminous flux of a sub-beam SB smaller than the effective pupil diameter of an objective lens 55, the sub-beam SB generated by the diffraction grating 53 is made incident obliquely against an optical axis of the objective lens 55. Subsequently, after transmitting through the objective lens 55, it is reflected by a recording medium 56, and even if it is made incident on the objective lens 55, it is not vignetted. In such a way, it is suppressed that the unbalance is generated in a tracking error signal, and the exact tracking can be executed.