A semiconductor device with less variations in transistor characteristics is provided. The semiconductor device includes: a first insulator; a first oxide over the first insulator; a first conductor and a second conductor over the first oxide; a first layer and a second layer which are in contact with a side surface of the first oxide; a second insulator over the first insulator, the first layer, the second layer, the first conductor, and the second conductor; a third insulator over the second insulator; a second oxide between the first conductor and the second conductor and over the first oxide; a fourth insulator over the second oxide; and a third conductor over the fourth insulator. Each of the first layer and the second layer includes a metal contained in the first conductor and the second conductor. The first insulator in a region in contact with the second insulator includes a region where a concentration of the metal is lower than that of the first layer or the second layer.