Provided is a semiconductor device that can be miniaturized in a simple process and that can prevent deterioration of electrical characteristics due to miniaturization. The semiconductor device includes an oxide semiconductor layer, a first conductor in contact with the oxide semiconductor layer, and an insulator in contact with the first conductor. Further, an opening portion is provided in the oxide semiconductor layer, the first conductor, and the insulator. In the opening portion, side surfaces of the oxide semiconductor layer, the first conductor, and the insulator are aligned, and the oxide semiconductor layer and the first conductor are electrically connected to a second conductor by side contact.