1. A music-piece classifying apparatus comprising: first means for converting audio data representative of a music piece into data components in respective different frequency bands for every unit time interval to generate time frequency data pieces assigned to the respective different frequency bands; second means for detecting, from the time frequency data pieces generated by the first means, each sustain region in which a data component in one of the frequency bands continues to occur during a reference time interval or longer; third means for calculating a feature quantity from at least one of (1) a number of the sustain regions detected by the second means and (2) magnitudes of the data components in the sustain regions; and fourth means for classifying the music piece in response to the feature quantity calculated by the third means.