A method and an apparatus are provided which are capable of evaluating a state
of a signal with a fewer number of samples compared with an error rate. A reproducing
signal obtained from a magneto-optical disk is subjected to maximum-likelihood
decode in a maximum-likelihood decoder, paths coupling with each other in the maximum-likelihood
decode is detected by an evaluation index generation unit and, at the same time,
a difference of likelihoods (path metrics) of those paths is obtained to adjust
a parameter of a recording and reproducing signal or a control signal of an optical
pickup based on the likelihood difference. The parameter is, for example, auto-tracking,
a gain of an auto-focusing loop, or an offset value, or a gain value of a reproducing
signal, a recording power, or a reproducing power intensity.