By operating a driving TFT in a saturation region, luminance is not easily reduced when an EL element is degraded. However, such problems occur as a high voltage, high power consumption, and heat generation. In the case of operating a driving TFT in a saturation region, luminance varies due to a variation of driving TFTs. In view of the aforementioned problems, a high current capacity TFT is used in the high gray-scale and a low current capacity TFT is used in the low gray-scale. The high current capacity TFT can supply a large current with a lower Vgs, therefore, it does not easily operate in a linear region even when Vds is lowered. Thus, a luminance is not reduced easily even when an EL element is degraded, and an operation at a low voltage is realized. The low current capacity TFT supplies current when high Vgs is applied. With high Vgs, an effect of variation in characteristics of TFTs, especially in Vth can be ameliorated.