Patent ID: 7573788

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 the recording layer 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 in the objective lens; a thickness unevenness detecting system that detects a thickness unevenness in a transparent resin layer of the recording medium provided nearest the objective lens; and 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, wherein a DC gain Gof of the objective lens moving mechanism is larger than a DC gain Got of the thickness unevenness correcting mechanism.