A minute transistor with low parasitic capacitance, high frequency characteristics, favorable electrical characteristics, stable electrical characteristics, and low off-state current is provided. A semiconductor device includes a semiconductor over a substrate, a source and a drain over the semiconductor, a first insulator over the source and the drain, a second insulator over the semiconductor, a third insulator in contact with a side surface of the first insulator and over the second insulator, and a gate over the third insulator. The semiconductor includes a first region overlapping with the source, a second region overlapping with the drain, and a third region overlapping with the gate. The length between a top surface of the third region of the semiconductor and a bottom surface of the gate is longer than the length between the first region and the third region.