As for a semiconductor device which is typified by a display device, it is an object to provide a highly reliable semiconductor device to which a large-sized or high-definition screen is applicable and which has high display quality and operates stably. By using a conductive layer including Cu as a long lead wiring, an increase in wiring resistance is suppressed. Further, the conductive layer including Cu is provided in such a manner that it does not overlap with the semiconductor layer in which a channel region of a TFT is formed, and is surrounded by insulating layers including silicon nitride, whereby diffusion of Cu can be prevented; thus, a highly reliable semiconductor device can be manufactured. Specifically, a display device which is one embodiment of a semiconductor device can have high display quality and operate stably even when the size or definition thereof is increased.