To provide a highly reliable semiconductor device that is suitable for miniaturization and an increase in density. The semiconductor device includes a first insulator over a substrate, a transistor including an oxide semiconductor over the first insulator, a second insulator over the transistor, and a third insulator over the second insulator. The first insulator and the third insulator have a barrier property with respect to oxygen and hydrogen. The second insulator includes an excess-oxygen region. The transistor is enclosed with the first insulator and the third insulator that are in contact with each other in an edge of a region where the transistor is positioned.