To provide an optical disk device capable of preventing a mistake in determination of the number of recording layers of an optical disk.

In an optical disk device, a movable lens for spherical aberration correction is fixed to a position shifted from a position where spherical aberration exhibits roughly best performance in a recording layer farthest from the side from which light beams are incident so as to come close to the position where the spherical aberration roughly exhibits best performance in a certain recording layer other than the recording layer farthest from the side from which the light beams are incident (for example, the position where the spherical aberration roughly exhibits best performance in the recording layer which is second farthest from the side from which the light beams are incident), and the number of S characters of a focus error signal is detected (refer to a step S10 and a step S30).