User is allowed to designate a desired mode defining the respective numbers of channels and mixing buses, and processing for mixing input signals of the number of channels corresponding to the designated mode is performed repetitively to generate signals for the individual buses. The time of arrival of the last step in the mixing processing for the number of channels, corresponding to the designated mode, is detected to output an accumulation result obtained at the last step, and new accumulation is started with a digital audio signal inputted at a step following the last step. Digital audio signals processed by a first signal processing circuit are stored into a memory and transmitted to a second signal processing circuit via a cascade-connection. The second signal processing circuit adds the audio signal, processed for each of the steps, to audio signals input via the cascade-connection and writes added signal into the memory.