From a storage device storing waveform sample data sampled at a given sampling rate, the waveform sample data are read out asynchronously with the given sampling rate and temporarily stored in an input buffer. The thus-stored waveform sample data are read out from the input buffer and converted to an inner sampling rate selected from among a plurality of predetermined inner sampling rates. Predetermined arithmetic processing is performed on the waveform sample data converted to the selected inner sampling rate. Then, the waveform sample data having undergone the predetermined arithmetic processing are converted to a predetermined output sampling rate and written into an output buffer. The thus-written waveform sample data are then sequentially read out at an sampling frequency corresponding to the output sampling rate, to thereby generate tone data in accordance with the output sampling frequency.