An evaluating device including: a Viterbi detector to perform bit detection by Viterbi detection from a reproduced signal in which bit information is reproduced; a metric difference calculator to calculate a metric difference between values of path metrics for a second path and a maximum likelihood path when at least an error pattern between the maximum likelihood path as a path surviving as a result of path selection by the Viterbi detector and the second path compared finally with the maximum likelihood path corresponds to one of a predetermined plural error patterns; and an evaluation valuecalculator to compare each of values of metric differences calculated by the metric difference calculator for each error pattern, with an individual threshold value obtained by dividing a Euclidean distance between the maximum likelihood path and the second path in each error pattern by a common value, and calculate a total number of values of the metric differences less than the threshold valueas an evaluation value.