In an apparatus capable of generating a tone by a plurality of different tone generation processing modes, the respective tone generation processing modes are arbitrarily assigned in units of tone generation channels so as to efficiently utilize the tone generation channels. In particular, as one of the tone generation processing modes, a waveform readout processing such as a PCM method is used. As another processing mode, a tone generation processing mode for converting an output obtained by mixing a modulation signal in a carrier signal at an arbitrary ratio in accordance with a predetermined functional relationship to generate a tone signal is used. In this mode, when the mixing ratio is set to be a predetermined value, the tone signal has a sine or cosine wave. Various tones such as a tone including a single sine or cosine wave component, a tone including harmonic components of high orders, and the like can be widely synthesized, and can be desirably assigned to the tone generation channels. The tone signals synthesized by the two tone generation processing modes are mixed at a predetermined mixing ratio which changes over time, and the mixed signal is output as a mixed tone signal, thereby allowing generation of a tone which changes complicatedly and dramatically.