To provide a transistor having a high on-state current. A semiconductor device includes a first insulator containing excess oxygen, a first oxide semiconductor over the first insulator, a second oxide semiconductor over the first oxide semiconductor, a first conductor and a second conductor which are over the second oxide semiconductor and are separated from each other, a third oxide semiconductor in contact with side surfaces of the first oxide semiconductor, a top surface and side surfaces of the second oxide semiconductor, a top surface of the first conductor, and a top surface of the second conductor, a second insulator over the third oxide semiconductor, and a third conductor facing a top surface and side surfaces of the second oxide semiconductor with the second insulator and the third oxide semiconductor therebetween. The first oxide semiconductor has a higher oxygen-transmitting property than the third oxide semiconductor.