PURPOSE: To recognize propriety in the adjustment state of contrast, bright, etc., as digital quantity by enabling recognizing the dynamic range of a circuit system in the video signal circuit of a displaying device which requires the input of a digital signal such as PDP, etc.

CONSTITUTION: A change-over circuit 4 which sets one of the adding processing or the subtracting processing of required data for data concerning the respective red, blue and green digital input video signals (Sr, Sg and Sb), a generating circuit 5 for addition/subtraction variable data as required data and an adder/ subtracter 7 which executes the addition or subtraction of input video signal data and addition/subtraction data by the generating circuit 5 for addition/ subtraction variable data based on setting by the change-over circuit are respectively provided at every digital video signal. Addition or subtraction is executed by varying addition/subtraction data so that the dynamic range of the circuit system concerning the respective digital video signals is recognized.