Patent ID: 7778129

Claim:
An optical disk apparatus for recording information on and reproducing information from an optical disk formed by sequentially laying a first recording layer, a second recording layer and a cover layer by irradiating a light beam from the cover layer side, the apparatus comprising: an objective lens for converging the light beam emitted from a light source to irradiate the optical disk; drive means for driving an objective lens in the direction of the optical axis of the light beam to move the focal position of the light beam; spherical aberration correcting means for correcting the spherical aberration produced by the light beam according to the first recording layer to be used for recording or reproduction of information; light receiving means for receiving reflected light of the light beam and generating a sum signal indicating the quantity of light of the reflected light and a focus error signal for outputting the signal level as a function of the distance from the focal position of the light beam to the first recording layer; recording layer detecting means for moving the focal point of the light beam closer to the first recording layer from the cover layer side and detecting first and second S-shaped waveforms in the focus error signal; and recording layer determining means for determining whether the second S-shaped waveform in the focus error signal indicates the first recording layer by continuing movement of the focal point of the light beam closer to the optical disk after the detection of the second S-shaped waveform by the recording layer detecting means for a predetermined overrun period, and determining that the second S-shaped waveform in the focus error signal indicates the first recording layer when another S-shaped waveform is not detected within the predetermined overrun period.