An optical disk apparatus is capable of reproducing information from an optical disk 10. The optical disk apparatus mainly includes an illuminating unit which is formed from a semiconductor laser 14, a collimator lens 15, a polarization light beam splitter (PBS) 16 and a quarter-wave plate 17, as well as an objective lens 18, that focuses a light beam onto the optical disk 10; a two-segment split photo detector 21 which detects the light beam reflected by the optical disk 10; and a signal processing circuit 27 which processes a signal obtained by the two-segment split photo detector 21 to output a signal for reproduction. Here, the optical disk 10 has a recording region on which recording information can be stored along a first line and a header field on which reproducible information is stored along a second line, the second line being shifted from the first line a predetermined distance, the illuminating unit focusing the light beam along the first line, and the two-segment split photo detector 21 having two detecting areas 22a and 22b on opposite sides of a line parallel to the reflected image of the first line position. The signal processing circuit 27 is capable of processing a difference and a sum signal by using values detected in the detecting areas 22a and 22b, and it outputs one of those to serve as a playback signal reflecting a characteristic of the optical disk 10.