An optical disc reproducing apparatus includes an A/D converter; an asymmetry compensator for detecting 4T sampling signals; a phase locked loop including a frequency detector that counts and detects run-length signals from the digital signals and compensates frequency errors of the digital signals; a binary module including a Viterbi decoder, a slicer, and a minimum T compensator that compensates the digital signal with a minimum signal having a unit cycle; an equalizer; an adaptive level error detector detecting a base level of the Viterbi decoder from both an input signal into the equalizer and an output signal from the Viterbi decoder, and computing a filtering coefficient of the equalizer from the base level; and a signal quality measurer measuring a jitter or an SbER of the digital signal.