A controller remote-controlling a digital mixer which performs signal processing to an input signal by a DSP to output the processed signal is provided with functions of: accepting the setting of level of a dummy signal; calculating a gain of the signal processing at each stage in the DSP based on a value of a parameter used for the remote controlling; calculating level that the dummy signal would have at a reference point selected by a reference point selection button if the dummy signal is assumed to be inputted to the DSP, based on the level of the dummy signal and the calculated gain; and displaying the calculated level in a level display portion.