A grand piano generates acoustic tones through vibrations of strings and sound board so that the acoustic tones are converted to analog audio signals at recording points over the sound board, and a group of waveform data sets are produced from the analog audio signal through sampling and analog-to-digital conversion; when electronic tones are generated, delay parameters and volume parameters are determined on the basis of differences between the recording points and tone radiating points occupied by loud speakers, the sets of waveform data series are sequentially read out from the group of waveform data sets and are modified with the delay parameters and volume parameters so that the electronic tones become close to the acoustic tones.