A display is conducted by using a time gray-scale system, in which one frame period is divided into a plurality of sub-frame periods, and a voltage applied to an EL element of a pixel is varied on a sub-frame period basis. Because of this, a display device is provided in which the fluctuations in brightness caused by an environment temperature of an EL element are suppressed with a gray-scale display method that is unlikely to be influenced by variations in characteristics of TFTs in a pixel portion and that is unlikely to be influenced by variations in a display period.