An optical head comprising a light source, objective lens, light splitting means, light receiving element, tracking error signal detection means, and spherical aberration detection means. Light splitting means has six regions divided by a first splitting line parallel to a longitudinal direction of information tracks, and second and third splitting lines perpendicular to the first line. Spherical aberration detection means compares a first focal point shift amount obtained by detecting a light spot size formed by focusing light fluxes, created by laser light passing through two regions between the second and third splitting lines, onto the light receiving element, and a second focal point shift amount obtained by detecting light spot size formed by focusing light fluxes, created by laser light passing through the four regions, on the outer side of second and third splitting lines, generating a spherical aberration error signal for detecting spherical aberration at the objective lens.