A voltage equal to the threshold value of a TFT (106) is held in capacitor unit (109). When a video signal is inputted from a source signal line, the voltage held in the capacitor unit is added thereto and a resultant signal is applied to a gate electrode of the TFT (106). Even when a threshold value is varied for each pixel, each threshold value is held in the capacitor unit (109) for each pixel. Thus, the influence of a variation in threshold value can be eliminated. Further, holding of the threshold value is conducted by only the capacitor unit (109) and a charge does not move at writing of a video signal so that a voltage between both electrodes is not changed. Thus, it is not influenced by a variation in capacitance value.