The semiconductor device includes a first insulating layer, a second insulating layer, an oxide semiconductor layer, and first to third conductive layers. The first conductive layer and the second conductive layer are connected to the oxide semiconductor layer. The second insulating layer includes a region in contact with the oxide semiconductor layer, and the third conductive layer includes a region in contact with the second insulating layer. The oxide semiconductor layer includes first to third regions. The first region and the second region are separated from each other, and the third region is located between the first region and the second region. The third region and the third conductive layer overlap with each other with the second insulating layer located therebetween. The first region and the second region include a region having a higher carbon concentration than the third region.