A semiconductor device having favorable and stable electrical characteristics is provided. The semiconductor device includes a first and a second transistor over an insulating surface. The first and the second transistors each include a first insulating layer, a semiconductor layer over the first insulating layer, a second insulating layer over the semiconductor layer, and a first conductive layer overlapping with the semiconductor layer with the second insulating layer interposed therebetween. The first insulating layer includes a convex first region that overlaps with the semiconductor layer and a second region that does not and is thinner than the first region. The first conductive layer includes a part over the second region where a lower surface of the first conductive layer is positioned below a lower surface of the semiconductor layer. The second transistor further includes a third conductive layer overlapping with the semiconductor layer with the first insulating layer interposed therebetween.