To easily observe an image under various environment and to clearly display the image by automatically adjusting tone of a color of the image display and contrast related to gradation in accordance with the change contents of brightness in a mixer device.

In the screen illuminance adjusting system of the mixer device, a plurality of palettes PL1 to PL3, which respectively correspond to a plurality of steps of the brightness of a color LCD screen, are beforehand stored in a storage means and a brightness change operating section is provided to change the brightness of the screen into High, Mid., and Low. When change in brightness is instructed by a user's operation of an operator NB of the brightness change operating section, the change contents are detected, brightness of a back light BL is changed in accordance with the detected change contents, palettes PL1 to PL3 of the steps corresponding to the detected change contents are selected from the storage means, tone of the color and luminance of a plotting are changed in accordance with the tone of the color of the selected palettes PL1 to PL3 and luminance control data LM and CH and adjustment is made to obtain the best contrast.