To solve such a problem that in a light emitting device, brightness irregularities caused by the fact that threshold values of TFT supplying a current to the light emitting element are uneven for each pixel, have been fetters for improving image quality of the light emitting device.

With a voltage equal to the threshold value of a TFT held in capacitor means, when a video signal is input from a source signal line, the voltage held in the capacitor means is added and applied to a gate electrode of the TFT. Even when thresholds are varied for each pixel, each threshold value is held in the capacitor means for each pixel, so that the influence of variations in the threshold can be eliminated. Furthermore, the threshold is stored only in the capacitor means and the voltage between two electrodes is not changed while a video signal is written, so that the variations in a capacitance value have no effect.