To reduce seek time when reproducing data in a RAM by changing a servo parameter so that it is the best at the header part in seeking operation.

A parameter optimization means 111 calculates a parameter value for achieving the best reproduction quality according to a jitter value being measured by a jitter measurement means 110 for each of header and data parts. More specifically, in the parameter optimization, a parameter where a jitter voltage value is minimized is determined so that the best reproduction quality can be obtained. The parameter at this time is called an optimum parameter. In the parameter, a servo parameter is included, and a focus offset, a waveform equalization counter (fc), the amount of servo boost, or the like is used. The parameter obtained in this manner is stored into a parameter storage part for updating and learning. By a parameter change-over means 112, a seeking operation starting signal from a control part 109 is used as a trigger for changing the parameter for the data part to that for the header part.