A semiconductor device with favorable electrical characteristics is provided. A highly reliable semiconductor device is provided. A semiconductor device with stable electrical characteristics is provided. A semiconductor device includes a first insulating layer, a second insulating layer, a semiconductor layer, and a first conductive layer. The semiconductor layer, the second insulating layer, and the first conductive layer are stacked in this order over the first insulating layer. The second insulating layer has a stacked-layer structure in which a first insulating film, a second insulating film, and a third insulating film are stacked in this order. The first insulating film, the second insulating film, and the third insulating film each contain an oxide. The first insulating film includes a portion in contact with the semiconductor layer. The semiconductor layer contains indium, gallium, and oxygen and includes a region with an indium content percentage higher than a gallium content percentage.