In an optical disk having multiple layers for which a correction of spherical aberration is required, a spherical aberration correcting element is set to a predetermined spherical aberration correction amount; a laser light source is turned ON; an objective lens is swept; when it is discriminated that the optical disk has two or more layers, or has more than two information recording planes, the spherical aberration correction amount is changed into a spherical aberration correction amount suitable for information recording planes of three, or more layers; and the objective lens is again swept so as to discriminate a total layer number thereof.