A digital mixer includes a MO switch and a MO fader in a signal transmission route from each input channel to a bus of a MIX_MINUS type, a signal of the input channel is supplied to the MIX bus without control on its level when the MO switch is OFF, while being supplied to the MIX bus after being controlled on its level by the MOL fader when the MO switch is ON. When the MO switch is changed from OFF to ON, a level adjustment amount by the MOL fader is set to a value so that a signal passing therethrough comes to have zero level.