1. An information recording and reproducing apparatus for calculating an identification signal by using Partial Response and Maximum Likelihood identification scheme, the apparatus comprising: a table which stores true patterns and ideal signals thereof, false patterns corresponding to the true patterns and ideal signals thereof, and Euclidean distance between the true pattern and the false pattern; a calculator which, when the identification signal coincides with any pattern of the table, calculates a difference between Euclidean distance between the ideal signal of the true pattern and a reproduction signal and Euclidean distance between the ideal signal of the false pattern and a reproduction signal; and an evaluation unit configured to evaluate a reproduction signal based on an average and standard deviation in distribution of the difference.