An object is to suppress decrease in luminance and appearance of flicker of a still image and to control a threshold voltage of a transistor for driving an EL element even in a state where the EL element continues to emit light for a certain period. An n-channel transistor and a p-channel transistor are provided as driving transistors for driving a light-emitting element, and a polarity of a potential which is supplied from a data line is reversed every given period and supplied to gates of the driving transistors in each pixel, whereby the threshold voltages of the driving transistors are controlled and change of luminance of the light-emitting element due to the threshold voltage shifts of the driving transistors can be reduced.