A method for rating an information recording medium according to the present invention includes the steps of: receiving a digital read signal, which has been generated based on an analog read signal representing information that has been read from the information recording medium, and shaping the waveform of the digital read signal; subjecting the shaped digital read signal to maximum likelihood decoding, thereby generating a binarized signal showing a result of the maximum likelihood decoding; and calculating the quality of the digital read signal based on the shaped digital read signal and the binarized signal. If the quality of the read signal is calculated by a PRML method in which a number of zero-cross portions are included in a merging path of a minimum difference metric, the quality is calculated by using only a state transition pattern in which only one zero-cross portion is included in a merging path of a non-minimum difference metric.