1. A music analysis apparatus comprising: a feature extractor configured to generate a time series of feature values from a sequence of notes which is designated as an object of analysis; and an analysis processor configured to compute an evaluation index value which indicates a probability that the designated sequence of notes is present in each of a plurality of reference music pieces by applying a probabilistic model to the time series of the feature values generated from the designated sequence of notes, the probabilistic model being generated by machine learning of the plurality of reference music pieces using time series of feature values obtained from the reference music pieces, wherein the feature extractor is configured to divide the designated sequence of notes along a time axis into a plurality of unit periods so as to generate each feature value from each unit period which contains one or more of notes belonging to one or more of pitch classes, and the feature extractor is configured to calculate each sum of durations of notes belonging to each pitch class in the unit period and configured to calculate the feature value of the unit period based on each sum of durations of notes belonging to each pitch class in the unit period.