1. A music-piece classifying apparatus comprising: first means for subjecting an audio signal representative of a music piece input from an input device to frequency analysis to generate frequency component data composed of respective frequency components corresponding to time, frequency band, and component intensity; second means for detecting, with respect to the frequency component data corresponding to a prescribed portion or whole of the music piece, the frequency components satisfying a prescribed condition as effective components for each block being an interval containing a first prescribed number of the frequency components of one frequency band in a time base direction, and for, in cases where a second prescribed number or more of the effective components of said one frequency band are present in the block, detecting a combination of the block and said one frequency band as a sustain region; 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) the component intensities of the frequency components in the sustain regions; and fourth means for classifying the music piece in response to the feature quantity calculated by the third means.