an optical disk apparatus adopted to play back an optical disk , includes a laser source to emit a laser beam to an optical disk , a condenser lens to condense the laser beam reflected by the optical disk , a photodetector irradiated by the condensed laser beam and including a main light receiving part and an auxiliary light receiving part disposed adjacently to the main light receiving part , and a signal processor to output a difference between an output of the main light receiving part and an output of the auxiliary light receiving part as a playback signal representing information recorded on an information recording layer of the optical disk .