In a digital mixer capable of forming a plurality of externally-input signals into multi-channel surround signals by a mixing bus section, a plurality of output channels are set which correspond to a multi-channel surround channel configuration and are connected to a monitoring speaker system. When the externally-input signals are multi-channel surround signals, the digital mixer permits a selection as to whether the externally-input signals are to be monitored. If the signals are to be monitored, they are patched or converted in such a manner that their surround channel configuration agrees with the surround channel configuration of monitoring output channels, and then the externally-input signals are distributed among the monitoring output channels. Further, the digital mixer permits another selection as to whether the multi-channel surround signals formed by the mixing bus section are to be monitored. If the signals are to be monitored, they are distributed among the monitoring output channels.