In a light emitting device, luminance irregularities caused by fluctuation in threshold of TFTs for supplying a current to EL elements among pixels hinder the light emitting device from improving the image quality. A voltage equal to the threshold of a TFT 110 is held in capacitor means 111 in advance. When a video signal is inputted from a source signal line, the voltage held in the capacitor means is added to the signal, which is then applied to a gate electrode of the TFT 110. Even when threshold is fluctuated among pixels, each threshold is held in the capacitor means 111 of each pixel, and therefore, influence of the threshold fluctuation can be removed. Since the threshold is stored in the capacitor means 111 alone and the voltage between two electrodes is not changed while a video signal is written, fluctuation in capacitance value has no influence.