As viewed from the front of an operation panel, adjustment sections for individual input channels are positioned on the operation panel close to a human operator, and a display, a control section for setting an equalizer process, compressor process etc. for one selected channel and a signal delivery control section for setting signal delivery to mixing buses for the selected channel are positioned, in a horizontal row remotely from the human operator. Operators in the delivery control section are varied in function in accordance with a selected operation mode. For example, in a mix send mode, a plurality of rotary operators are set to function as send level adjustment operators for the respective input channels so as to adjust send levels to a given mixing bus, while, in a mix master mode, the rotary operators are set to function as output gain adjustment operators for the respective mixing buses.