number of pixels which are to be lightened on the entire display screen the luminance controller 15 reads out data indicative of a control condition of luminance from the reference table 16 during the refreshing period referring to this data together with the contents of the pixel counter 12 the contents of the luminance register 14 and the value of a photo sensor 19 which detects the brightness or luminance of the interior of a room the luminance controller 15 produces a luminance control signal which makes the brightness of the display screen optimum as shown in fig2 the luminance control signal is d a converted by the d a converter 17 and applied to the display unit 11 the luminance control condition provided by the reference table 16 may be designated through a host computer 0 and the display processor 1 from a keyboard 20 in accordance with the user 39 s taste or desire in the case where color data r g b for each pixel is directly stored in the refresh buffer 2 means constituted by for example coefficients 31 33 multipliers 34 36 and adder 37 for calculating the luminance i p for the pixel p in accordance with the equation is provided between the color look up table 7 and the connection point of adder 13 and pixel counter 12 as shown in fig3 in the equation 1 r p g p and b p are color data for the pixel p and k r k g and k b are coefficients in the foregoing embodiment the luminance of the display screen has been determined for each refresh cycle to dynamically control the luminance of the display screen in conformity with the surrounding condition on the other hand fig5 shows another embodiment of the present.