To provide a microscopic transistor, a transistor with small parasitic capacitance, a transistor with a high frequency characteristic, and a semiconductor device including the transistor.SOLUTION: A transistor includes an oxide semiconductor 406b, a first conductor 404, a second conductor 416a1, a third conductor 416b, a first insulator 406c, and a second insulator 410. The first conductor includes a region where the first conductor and the oxide semiconductor overlap with each other through the first insulator. The second insulator includes an opening. In the opening, a side surface of the second insulator and a side surface of the first conductor include a region overlapping with each other through the first insulator. A part of a surface of the second conductor and a part of a surface of the third conductor are in contact with the first insulator in the opening. The oxide semiconductor includes a region where the oxide semiconductor and the second conductor overlap with each other and a region where the oxide semiconductor and the third conductor overlap with each other.SELECTED DRAWING: Figure 1