An apparatus for detecting a tracking error signal includes an optical detector to receive light beams reflected from an optical disc and having a plurality of detecting regions; and a phase difference extracting unit to extract a phase difference between a pair of optical signals output from the optical detector, wherein the phase difference extracting unit repeatedly moves a first graph representing one optical signal from among the pair of optical signals by a predetermined distance with respect to a second graph representing another optical signal from among the pair of optical signals, the first and second graph represent optical signal values with respect to t-axis, adds differences between optical signal values of the first graph and optical signal values of the second graph corresponding to predetermined positions of the t-axis, the adding differences is repeated for each of the repeatedly moved positions of the first graph, and outputs a moved distance of a position where the added value is smallest as a phase difference.