The present invention is capable of selectively reproducing an information signal from an optical disk with any of a plurality of types having different track pitches of recording tracks, that is, capable of selectively reproducing an information signal from an optical disk having a low or high recording density by determining the type of the optical disk, that is, determining whether a mounted optical disk is a low-recording-density optical disk or a high-recording-density optical disk. The present invention deliberately generates aberration in an optical beam by controlling a voltage applied to a liquid-crystal device in order to vary the diameter of the optical beam in accordance with the determined type of the optical disk. The present invention is thus characterized in that it is applicable to an optical pickup apparatus for changing the diameter of an optical beam in accordance with whether a mounted optical disk is a low-recording-density optical disk or a high-recording-density optical disk, as well as applicable to an optical-disk reproducing apparatus and an optical-disk recording & reproducing apparatus that employ the optical pickup apparatus.