A mixer includes a plurality of channel modules, and audio signals having processed in the individual channel modules are output from a main output. A temporary channel module is created and made usable in response to a preview start instruction for previewing a desired one of the channel modules as a preview-object channel module. The same audio signal as input to the preview-object channel module is input to the temporary channel module. The temporary channel module performs signal processing on the input audio signal independently of the preview-object channel module, and the audio signal having been subjected to the signal processing is output to a monitor output. Thus, a user is allowed to change a signal processing parameter value to be applied to an audio signal and preview the changed result, without influencing the main output.