A transistor with excellent electrical characteristics (e.g., on-state current, field-effect mobility, or frequency characteristics) is provided. The transistor includes an oxide semiconductor layer including a channel formation region, a first gate electrode, a second gate electrode, a source electrode, and a drain electrode. The oxide semiconductor layer is between the first gate electrode and the second gate electrode. The oxide semiconductor layer has a pair of side surfaces in contact with the source electrode and the drain electrode and includes a region surrounded by the first gate electrode and the second gate electrode without the source electrode and the drain electrode interposed therebetween.