A self-aligned transistor including an oxide semiconductor film, which has excellent and stable electrical characteristics, is provided. A semiconductor device is provided with a transistor that includes an oxide semiconductor film, a gate electrode overlapping with part of the oxide semiconductor film, and a gate insulating film between the oxide semiconductor film and the gate electrode. The oxide semiconductor film includes a first region and second regions between which the first region is positioned. The second regions include an impurity element. A side of the gate insulating film has a depressed region. Part of the gate electrode overlaps with parts of the second regions in the oxide semiconductor film.