Once a human operator operates a fader knob of a desired channel downwardly toward a predetermined position, resistance force against the fader knob is generated. As the human operator further lowers the fader knob beyond another predetermined position against the resistance force, the channel is set in a CUE-ON state, so that an audio signal (pre-fader signal) of the channel can be test-listened to as a CUE signal. Then, once the human operator operates the fader knob upwardly, the CUE-ON state of the signal is canceled, so that a sound volume level of the channel can be controlled in accordance with an operating position of the fader knob.