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 value calculator 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 value as an evaluation value.