The present invention aims to perform high-accuracy focus control in an
optical disk drive, even if a solid immersion lens (SIL) or a solid
immersion mirror (SIM) is set between an objective lens and the optical
disk in order to increase a numerical aperture, by accurately detecting a
change of the distance between an SIL or SIM and an optical disk.
The present invention comprises light detection means (8) for detecting the
light incoming to an SIL (2) or an SIM from an objective lens (1) at an
incidence angle at which a numerical aperture is larger than 1 and
reflected from a plane of the SIL (2) or SIM facing an optical disk, light
detection means 10 for detecting the light incoming to the objective lens
(1), comparison means (11) for finding a ratio between light amounts of
the light detected by light detection means (8) and (10), and control
means (11) for controlling operations of a focus actuator in accordance
with the ratio.
