frequency characteristics of the equalizer eq 1 and equalizer eq 2 in fig2 the frequency is plotted on the horizontal axis and the gain is plotted on the vertical axis the plot for when equalizer eq 1 does not boost the rf signal is shown by the dotted line as can be seen the frequency characteristics are flattened accordingly when the signal supplied from the equalizer eq 1 is digitized and a jitter amount based on all ts is detected from the phase difference between the digitized signal and a clock signal frequency characteristics as shown by the dotted line in fig5 are obtained calculating an intermediate value between two points of generally equal jitter amounts make it possible to precisely determine an fs offset amount and or ts offset amount which minimizes the jitter amount and thus to adjust the optical pickup 12 this optimizes the focus of the optical pickup 12 and reduces the laser power of the lf to obtain a desired error rate further because the equalizer eq 2 boosts a high frequency region near 3t of the rf signal by a predetermined value 20 db a reproduction signal is reliably decoded thereby reducing the error rate thus instead of fixing the boost amount in the equalizer 16 in accordance with the present embodiment the boost amount is switched between zero and a predetermined value and an rf signal having flat delay characteristics is used during adjustment thereby making it possible to easily optimize the optical pickup in this embodiment.