It is an object to improve operation characteristics and reliability of a semiconductor device. A semiconductor device which includes an island-shaped semiconductor film having a channel-formation region, a first low-concentration impurity region, a second low-concentration impurity region, and a high-concentration impurity region including a silicide layer; a gate insulating film; a first gate electrode overlapping with the channel-formation region and the first low-concentration impurity region with the gate insulating film interposed therebetween; a second gate electrode overlapping with the channel-formation region with the gate insulating film and the first gate electrode interposed therebetween; and a sidewall formed on side surfaces of the first gate electrode and the second gate electrode. In the semiconductor device, a thickness of the gate insulating film is smaller in a region over the second low-concentration impurity region than in a region over the first low-concentration impurity region.