In a semiconductor device including an oxide semiconductor, a change in electrical characteristics is inhibited and reliability is improved. The semiconductor device is manufactured by a method including first to fourth steps. The first step includes a step of forming an oxide semiconductor film, the second step includes a step of forming an oxide insulating film over the oxide semiconductor film, the third step includes a step of forming a protective film over the oxide insulating film, and the fourth step includes a step of adding oxygen to the oxide insulating film through the protective film. In the first step, the oxide semiconductor film is formed under a condition in which an oxygen vacancy is formed. The oxygen from the oxide insulating film fills the oxygen vacancy after the fourth step.