To provide a novel method for manufacturing a semiconductor device. To provide a method for manufacturing a highly reliable semiconductor device at relatively low temperature. The method includes a first step of forming a first oxide semiconductor film in a deposition chamber and a second step of forming a second oxide semiconductor film over the first oxide semiconductor film in the deposition chamber. Water vapor partial pressure in an atmosphere in the deposition chamber is lower than water vapor partial pressure in atmospheric air. The first oxide semiconductor film and the second oxide semiconductor film are formed such that the first oxide semiconductor film and the second oxide semiconductor film each have crystallinity. The second oxide semiconductor film is formed such that the crystallinity of the second oxide semiconductor film is higher than the crystallinity of the first oxide semiconductor film.