An optical pickup apparatus for compensating for the deviation in the thickness of the optical disk and method therefor. The optical pickup includes a light source which emits light, an optical path changer which changes a proceeding path of incident light, an objective lens which focuses incident light to form a light spot on an optical disk, a light splitter, provided on the optical path between the light source and the optical path changer, which splits incident light into at least two light beams including first and second light beams so that at least two light spots including a main light spot having no aberration and an auxiliary light spot having a predetermined spherical aberration are formed on the optical disk, a photodetector including first and second light receiving portions which respectively receive the first and second incident light beams reflected by the optical disk and passing through the optical path changer, and a signal processing portion, including a delay which delays the phase of a second signal in order to synchronize a first signal and the second signal which are detected and optoelectrically converted by the first and second light receiving portions, to compensate for the deviation in the thickness of the optical disk.