Based on the understanding that time-varying characteristics of a tone element, such as an amplitude and pitch, in waveform data acquired through a live performance of a musical instrument include a variation component intended or controllable by a human player and a variation component not intended or non-controllable by the human player, the present invention allows the two components to be adjusted/controlled separately and independently of each other, so as to achieve effective and high-quality control. Discrete variation value train is acquired for at least one particular tone element in original waveform data, and the acquired variation value train is separated, in accordance with a time constant factor, into a "swell" value train of a relatively great time constant and a "fluctuation" value train of a relatively small time constant. The "swell" value train and "fluctuation" value train are variably controlled independently of each other. In this way, high-quality control can be performed on tone elements, such as an amplitude and pitch, included in the sampled waveform data.