A transistor includes a first insulator over a substrate; a first oxide thereover; a second oxide in contact with at least part of the top surface of the first oxide; a first conductor and a second conductor each in contact with at least part of the top surface of the second oxide; a third oxide that is over the first conductor and the second conductor and is in contact with at least part of the top surface of the second oxide; a second insulator thereover; a third conductor which is over the second insulator and at least part of which overlaps with a region between the first conductor and the second conductor; and a third insulator which is over the third conductor and at least part of which is in contact with the top surface of the first insulator. The thickness of a region of the first insulator that is in contact with the third insulator is less than the thickness of a region of the first insulator that is in contact with the first oxide.