An object of the invention is to suppress degradation in image quality of a liquid crystal display device which performs display by field sequential method and to reduce power consumption of a backlight. The highest brightness of a first color light in a pixel region is detected. Gamma correction is performed so that transmittance of a pixel of the region displaying the highest brightness of the first color light is set to maximum and transmittance of other pixel of the region is decreased in accordance with lowering of the first color light intensity, and the region is irradiated with the highest brightness of the first color light. Similarly, a second color light is irradiated in another region concurrently with irradiation of the first color, whereby input of an image signal and lighting of the backlight are performed simultaneously in every region of the pixel portion.