To provide a semiconductor device with favorable characteristics and a novel structure.

The semiconductor device includes: an oxide semiconductor layer; a source electrode and a drain electrode electrically connected to the oxide semiconductor layer; a gate insulating layer covering the oxide semiconductor layer, the source electrode, and the drain electrode; and a gate electrode over the gate insulating layer. The source electrode and the drain electrode include an oxide region formed by oxidizing a side surface thereof. The oxide region of the source electrode and the drain electrode is preferably formed by plasma treatment with a high frequency power of 300 MHz to 300 GHz and a mixed gas of oxygen and argon.