Patent ID: 7362668

Claim:
An optical head unit comprising: a light source that supplies light of a predetermined wavelength; an objective lens that focuses the light from the light source to one of recording layers of a recording medium; an objective lens moving mechanism that moves the objective lens in the optical axis direction and in the direction intersecting a signal mark string formed in the recording medium; a defocus detecting system that detects a defocus generated when light converged to said one of recording layers of the recording medium by the objective lens is not focused at a predetermined position; a thickness unevenness detecting system including at least two photo detecting regions that detects unevenness (spherical aberration) in a transparent resin layer of the recording medium provided nearest the objective lens; a thickness unevenness correcting mechanism that changes a focusing characteristic of light impinging upon the objective lens from the light source based on a change in the thickness of the transparent resin layer of the recording medium detected by the thickness unevenness detecting system; and a defocus correcting mechanism that corrects a defocus detected by the defocus detecting system, wherein when information successively recorded in two recording layers of the recording medium are to be reproduced, the objective lens is located in a position against one of these targeted recording layers to which a change is made, and a tracking servo, the thickness unevenness detecting system, the thickness unevenness correcting mechanism, and a focusing servo are turned off, and upon confirmation of an output of a photo detector that the objective lens has been moved to a just-on-focus position against the next one of the targeted recording layers through a defocus correction control of the objective lens moving mechanism, the thickness unevenness correcting mechanism is activated to locate the objective lens on the next one of the targeted recording layers.