An optical head comprising a light source, an object lens, a light splitting means, a light receiving element, a tracking error signal detecting means, and a spherical aberration detecting means, wherein the light splitting means has six regions divided by a first splitting line parallel to the longitudinal direction of information tracks and by second and third splitting lines perpendicular to the first splitting line, and the spherical aberration detecting means compares the deviation of a first focus point obtained from a light flux passed through two regions disposed between second and third splitting lines with the deviation of a second focus point obtained from a light flux passed through four regions disposed on the outer side of the second and third splitting lines to generate a spherical aberration error signal for detecting a spherical aberration generated at the object lens.