A personal computer generates first gray level signals each dot of which is represented by N bits (where N is an integer larger than or equal to 2) representing 2N (the Nth. power of 2) gray levels. The first gray level signal is supplied to a CRT display, each dot of which displays 2N (the Nth. power of 2) gray levels. Now, the CRT is replaced by an LCD, each dot of which displays 2M (the Mth. power of 2) gray levels. An algorithm converts the first gray level signal of N bits representing 2N gray levels to second gray level signals representing 2M gray levels. The second gray level signals are applied to the LCD. The invention solves a problem raised when an algorithm is used to generate an image for LCDs and that image is expanded in a vertical direction.