A semiconductor device including a transistor having low leakage current between the drain and the gate is provided. The semiconductor device includes an insulating film provided so as to cover a corner of the first conductor and a second conductor provided so as to overlap with a corner of the first conductor with the insulating film provided therebetween. Variation in the thickness of the insulating film can be prevented by making the first conductor have a rounded corner. Furthermore, concentration of electric field due to the corner of the first conductor can be relaxed. Thus, the current leakage between the first conductor and the second conductor can be reduced.