An object is to provide a semiconductor device with a novel structure. A semiconductor device includes a first transistor, which includes a channel formation region provided in a substrate including a semiconductor material, impurity regions, a first gate insulating layer, a first gate electrode, and a first source electrode and a first drain electrode, and a second transistor, which includes an oxide semiconductor layer over the substrate including the semiconductor material, a second source electrode and a second drain electrode, a second gate insulating layer, and a second gate electrode. The second source electrode and the second drain electrode include an oxide region formed by oxidizing a side surface thereof, and at least one of the first gate electrode, the first source electrode, and the first drain electrode is electrically connected to at least one of the second gate electrode, the second source electrode, and the second drain electrode.