A waveshape memory stores a full waveshape of a tone from the start to the end of sounding of the tone or a portion thereof in plural periods. A tone wave signal produced by reading this waveshape memory is applied to a tone color circuit where its tone color is changed. The tone wave signal whose tone color has been changed and the tone wave signal whose tone color has not been changed are both multiplied with respective coefficients whereby these tone wave signals are weighted. The weighted tone wave signals are added together to provide a mixed tone signal. By controlling the coefficients, the tone color imparted on the mixed signal is variously determined. The coefficients for the tone color control are provided in accordance with key scaling, key touch or operation states of control knobs. Thus tone signals exhibiting a variety of tone color changes are obtained using not so many wave memories.