In the sound source system, the waveform sampled from the sounds of keys pressed down with two or more kinds of intensity are synthesized in a specified rate, thereby forming the waveform of the sounds of keys pressed down with the other kind of intensity. Thus, almost the actual instrumental deep sounds can be reproduced. The sound waveform sampled from weak key presses is passed through a low pass filter, and the filtered waveform composed only of low order harmonic components is transformed to weak press data in the pulse code modulation system for the storage. Similarly, the sound waveform sampled from strong key presses are passed through a high pass filter, thereby removing the low order harmonic components. The filtered waveform composed only of the high order harmonic components are transformed to strong press data in the pulse code modulation system for the storage. The high pass filter can remove the frequency components which pass through the low pass filter. By mixing the weak and strong press data in the rate determined by the key press intensity, the sound waveform signal of optional key press intensity is formed.