A semiconductor device with favorable electrical characteristics, a semiconductor device with stable electrical characteristics, or a highly reliable semiconductor device or display device is provided. A first insulating layer and a first conductive layer are stacked over a first region of a first metal oxide layer. A first layer is formed in contact with a second metal oxide layer and a second region of the first metal oxide layer that is not overlapped by the first insulating layer. Heat treatment is performed to lower the resistance of the second region and the second metal oxide layer. A second insulating layer is formed. A second conductive layer electrically connected to the second region is formed over the second insulating layer. Here, the first layer is formed to contain at least one of aluminum, titanium, tantalum, and tungsten.