An apparatus is provided in which a parameter of Viterbi decoding is adjusted on the basis of a metric difference for evaluation of PRML signal quality to increase the error rate even when using a medium on which recording is performed under a large-distortion condition. When a reproduced signal is binarized by maximum likelihood decoding such that a state transition sequence most probable to be correct of m kinds of state transition sequences from a first state at time n-1 to a second state at time n is selected, a metric difference between the most probable state transition sequence and a second most probable state transition sequence at a predetermined time is computed, and a parameter (a reference amplitude value or a metric) of a maximum likelihood decoder is corrected on the basis of the metric difference to optimize maximum likelihood decoding operation.