Mixer apparatus includes: a plurality of input channels, mixing buses and output channels; a monitor section that outputs, for a monitoring purpose, signals of one or more of the channels selected in response to monitoring selecting operation; and a plurality of effecters. For each of the plurality of effecters, setting is performed to insert the effecter in a designated one of the input or output channels, to allow the inserted effecter to be used in the designated channel. The mixer also permits editing of a selected one of the effecters while causing an editing screen for the selected effecter to be displayed. When any one of the effecters has been selected for editing while the mixer is set in the interlock mode, control is performed to automatically output, for the monitoring purpose, the signal of the channel having the selected effecter inserted therein.