In an RGBW-type liquid crystal display device, luminance is improved by the addition of W sub-pixels while an image is displayed without any change in chromaticity of halftones. Digital corrected values of red, green and blue are obtained by adding a predetermined digital value for driving a W sub-pixel to each of RGB digital values which correspond respectively to pixels of an acquired image. A converting calculation is effected on the digital corrected values such that the ratio of these digital corrected values for red, green and blue is made equal to the ratio of the red, green and blue digital values corresponding to the pixels of said acquired image. The RGBW sub-pixels are driven with the converted values and the predetermined digital value of driving W sub-pixel to thereby display an image.