Diffraction element and optical disk device

The present invention provides a use of a diffracted beam, and no diffracted beam, the tracking offset can be canceled, while preventing the occurrence of the astigmatic diffraction element; and an optical disk apparatus using the diffractive element. A diffraction element (8) having a plurality of opposed grooves on the surface of the surface portions (81) and the protrusions (82) grating regions (86) alternately arranged along the groove portion (81) on both sides of the longitudinal direction of flat flat area (87, 88), the center position in the depth direction (87, 88) and a groove is made (81) of the flat area of ​​the same height. Thus, by arranging the diffraction element so that the groove portion (81) located within the cross-sectional area smaller than the area of ​​the beam, even when a (3) generate element beams using, as a flat region (87, 88) and the groove is made the center position of the same height (81) in the depth direction of the unit, it is possible to prevent the diffraction element (8) astigmatism occurs.