One of the objects is to improve display quality by reduction in malfunctions of a circuit. In a driver circuit formed using a plurality of pulse output circuits having first to third transistors and first to fourth signal lines, a first clock signal is supplied to the first signal line; a preceding stage signal is supplied to the second signal line; a second clock signal is supplied to the third signal line; an output signal is output from the fourth signal line. Duty ratios of the first clock signal and the second clock signal are different from each other. A period during which the second clock signal is changed from an L-level signal to an H-level signal after the first clock signal is changed from an H-level signal to an L-level signal is longer than a period during which the preceding stage signal is changed from an L-level signal to an H-level signal.