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.