To provide a semiconductor device with favorable electrical characteristics. To provide a method for manufacturing a semiconductor device with high productivity. To reduce the temperatures in a manufacturing process of a semiconductor device. An island-like oxide semiconductor layer is formed over a first insulating film; a second insulating film and a first conductive film are formed in this order, covering the oxide semiconductor layer; oxygen is supplied to the second insulating film through the first conductive film; a metal oxide film is formed over the second insulating film in an atmosphere containing oxygen; a first gate electrode is formed by processing the metal oxide film; a third insulating film is formed, covering the first gate electrode and the second insulating film; and first heat treatment is performed. The second insulating film and the third insulating film each include oxide. The highest temperature in the above steps is 340° C. or lower.