To give favorable electrical characteristics to a semiconductor device. To provide a semiconductor device in which a change in electrical characteristics is suppressed. To provide a highly reliable semiconductor device. The semiconductor device includes a first insulating layer; a second insulating layer including an opening portion, over the first insulating layer; a semiconductor layer over the first insulating layer; a source electrode and a drain electrode that are apart from each other in a region overlapping with the semiconductor layer; a gate electrode overlapping with the semiconductor layer; and a gate insulating layer between the semiconductor layer and the gate electrode. The first insulating layer includes oxide, and the opening portion of the second insulating layer is positioned inside the semiconductor layer when seen from a top surface side and at least part of the opening portion is provided to overlap with the gate electrode.