Audio data representative of a music piece is converted 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. From the generated time frequency data pieces, detection is made as to each sustain region in which an effective data component in one of the frequency bands continues to occur during a reference time interval or longer. A feature quantity is calculated from at least one of (1) a number of the detected sustain regions and (2) magnitudes of the effective data components in the detected sustain regions. The music piece is classified in response to the calculated feature quantity.