To suppress a malfunction of a circuit due to deterioration in a transistor. In a transistor which continuously outputs signals having certain levels (e.g., L-level signals) in a pixel or a circuit, the direction of current flowing through the transistor is changed (inverted). That is, by changing the level of voltage applied to a first terminal and a second terminal (terminals serving as a source and a drain) every given period, the source and the drain are switched every given period. Specifically, in a portion which successively outputs signals having certain levels (e.g., L-level signals) in a circuit including a transistor, L-level signals having a plurality of different potentials (L-level signals whose potentials are changed every given period) are used as the signals having certain levels.