If regeneration is not performed normally when data is regenerated from a medium, a read retry processing unit changes the intensities of the laser beam and of the regeneration magnetic field respectively to prearranged stages to perform the regeneration again. A regeneration condition change unit remembers the regeneration sector number for one read command from the higher order. When retry occurs, the regeneration condition change unit counts the number of the sectors for each stage which can be regenerated at each stage, and by providing a coefficient for each stage in advance, changes the default regeneration condition using the sum of products of the coefficients and the number of the counted regenerable sectors.