In order to play waveform data back at a variable performance tempo by using waveform data which complies with a desired reference tempo, the present invention performs a timeline-expansion/contraction control on the waveform data to be played back, according to the relationship between the performance tempo and the reference tempo. The present invention also determines whether to limit the playback of the waveform data according to the relationship between the performance tempo and the reference tempo. In the case that playback is to be limited, the present invention stops playback of the waveform data, or reduces the resolution of playback processing and continues playback of the waveform data. The present invention stops playback of the waveform data when, for example, the relationship between the performance tempo and the reference tempo is a relationship in which the waveform data would be played back at a performance tempo which would cause a processing delay or a deterioration of sound quality. As a result, it is possible to preemptively prevent a system freeze and solve problems such as the generation of music which has a slower tempo than the desired performance tempo, or the generation of music which includes the intermittent cutting out of sound due to noise, or a significant reduction to sound quality.