A highly reliable semiconductor device which includes an oxide semiconductor is provided. Alternatively, a transistor having normally-off characteristics which includes an oxide semiconductor is provided. The transistor includes a first conductor, a first insulator, a second insulator, a third insulator, a first oxide, an oxide semiconductor, a second conductor, a second oxide, a fourth insulator, a third conductor, a fourth conductor, a fifth insulator, and a sixth insulator. The second conductor is separated from the sixth insulator by the second oxide. The third conductor and the fourth conductor are separated from the sixth insulator by the fifth insulator. The second oxide has a function of suppressing permeation of oxygen as long as oxygen contained in the sixth insulator is sufficiently supplied to the oxide semiconductor through the second oxide. The fifth insulator has a barrier property against oxygen.