Patent ID: 7145846

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 guide groove and a signal mark string preliminarily 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 to 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, the thickness unevenness correcting mechanism being operated with a smaller gain relative to a servo gain of a DC level of the frequency characteristic of a defocus correcting control circuit.