In a display device having a light emitting element, an accurate setting operation needs much time, unless a current value of the signal current (video signal) is set to high value. On the contrary, the driving current value for causing a light emitting element to emit light is very small. Therefore, it is difficult to perform an accurate setting operation. However, according to the present invention, the current source circuit includes plural transistors. The plural transistors are connected in parallel when the setting operation is performed on the current source circuit, whereas the plural transistors are connected in series when the light emitting element is caused to emit light. Further, the speed of the setting operation is increased because a light emitting element is capable of emitting light with a constant luminance and a current value to set up a current source circuit is higher than a driving current value when a light emitting element emits light.