Patent ID: 7274629

Claim:
An optical head unit, comprising: a light source configured to supply light of a predetermined wavelength; an objective lens configured to focus light from the light source to a recording layer of a recording medium; a defocus detecting system configured to detect a defocus generated when light converged to the recording medium by the objective lens is not focused at a predetermined position; a thickness unevenness detecting system configured to detect a thickness unevenness in a transparent resin layer of the recording medium provided nearest the objective lens, the thickness unevenness detecting system including at least two photo detecting regions; a thickness unevenness correcting mechanism configured to change focusing characteristic of light impinging upon the objective lens from a light source based on a change in thickness of the transparent resin layer of the recording medium detected by the thickness unevenness detecting system; and a defocus correcting mechanism configured to correct a defocus detected by the defocus detecting system, wherein the focused recording layer of the objective lens is changed by: turning off a thickness unevenness correction control; turning off a defocus correction control; moving the objective lens; turning on the defocus correction control; and turning on the thickness unevenness correction control.