A mixing system for supplying signals of a plurality of input channels to a plurality of mixing buses (32-1 to 32-24) of a mixing system via corresponding input channel adjustment sections of the mixing system and outputting the signals, mixed via individual ones of the mixing buses (32-1 to 32-24), as signals of a plurality of MIX output channels of the mixing system via output channel sections (36-1 - 36-24) of the mixing system corresponding to the mixing buses (32-1 to 32-24), the signals of the plurality of MIX output channels output via the output channel sections (36-1 - 36-24) being supplied to a main output section (44, 46), said mixing system comprising: bus selection means for selecting one mixing bus (32-k) to be subjected to adjustment from among said plurality of mixing buses (32-1 to 32-24), said bus selection means including a MIX bus selection key; send level adjusting means for adjusting send levels from individual ones of said input channel adjustment sections to the selected one mixing bus (32-k), via send level adjustment operators (64-1 to 64-24) provided in corresponding relation to said input channels; and monitoring channel selection means for selecting, as a monitoring signal, a signal of any one of said input channel adjustment sections or said output channel sections (36-1 - 36-24), the monitoring signal being supplied to a monitoring output section different from the main output section (44, 46), said monitoring channel selection means including a CUE key for manually selecting said signal as a monitoring signal, characterized in that said mixing system control method further comprises: means for selecting an ON or OFF state of an interlocking function that is a function for interlocking the monitoring channel selection means to the MIX bus selection key in such a manner that, when the one mixing bus (32-k) is selected by operating said MIX bus selection key while the interlocking function is in the ON state, the monitoring channel selection means automatically select, without operation of said CUE key, a signal of the output channel section (36-k) corresponding to the selected one mixing bus (32-k)as the monitoring signal being supplied to the monitoring output section and when the one mixing bus (32-k) is selected by said bus selection means while the interlocking function is in the OFF state, a signal of the output channel section (36-k) corresponding to the selected one mixing bus (32-k) is not automatically selected as the monitoring signal being supplied to the monitoring output section. bus selection means for selecting one mixing bus (32-k) to be subjected to adjustment from among said plurality of mixing buses (32-1 to 32-24), said bus selection means including a MIX bus selection key; send level adjusting means for adjusting send levels from individual ones of said input channel adjustment sections to the selected one mixing bus (32-k), via send level adjustment operators (64-1 to 64-24) provided in corresponding relation to said input channels; and monitoring channel selection means for selecting, as a monitoring signal, a signal of any one of said input channel adjustment sections or said output channel sections (36-1 - 36-24), the monitoring signal being supplied to a monitoring output section different from the main output section (44, 46), said monitoring channel selection means including a CUE key for manually selecting said signal as a monitoring signal, characterized in that said mixing system control method further comprises: means for selecting an ON or OFF state of an interlocking function that is a function for interlocking the monitoring channel selection means to the MIX bus selection key in such a manner that, when the one mixing bus (32-k) is selected by operating said MIX bus selection key while the interlocking function is in the ON state, the monitoring channel selection means automatically select, without operation of said CUE key, a signal of the output channel section (36-k) corresponding to the selected one mixing bus (32-k)as the monitoring signal being supplied to the monitoring output section and when the one mixing bus (32-k) is selected by said bus selection means while the interlocking function is in the OFF state, a signal of the output channel section (36-k) corresponding to the selected one mixing bus (32-k) is not automatically selected as the monitoring signal being supplied to the monitoring output section. means for selecting an ON or OFF state of an interlocking function that is a function for interlocking the monitoring channel selection means to the MIX bus selection key in such a manner that, when the one mixing bus (32-k) is selected by operating said MIX bus selection key while the interlocking function is in the ON state, the monitoring channel selection means automatically select, without operation of said CUE key, a signal of the output channel section (36-k) corresponding to the selected one mixing bus (32-k)as the monitoring signal being supplied to the monitoring output section and when the one mixing bus (32-k) is selected by said bus selection means while the interlocking function is in the OFF state, a signal of the output channel section (36-k) corresponding to the selected one mixing bus (32-k) is not automatically selected as the monitoring signal being supplied to the monitoring output section.