An optical disk recording/reproducing apparatus comprising a chip whose size is reduced by using a microfabrication process and having an improved detection accuracy. The amplitude of the header partof a reproduced signal detected by an optical pickup (1) is adjusted, and the reproduced signal is digitized. Detection circuits (9 to 12) peak-detect or bottom-detect the produced signal in first andsecond sections of the header part. Subtraction operating units (13, 14) detect amplitude information on the first and second sections from the detection values detected by the detection circuits. Asubtraction unit (15) calculates the difference in amplitude between the first and second sections from the amplitude information and outputs the amplitude difference as an off-track error signal. Thedetection circuits (9 to 12) perform detection in the first and second sections and are controlled to hold detection in the other sections than the first and second sections.