To provide an oxide semiconductor film including a low-resistance region, which can be applied to a transistor. To provide a transistor including the oxide semiconductor film, which can perform at high speed. To provide a high-performance semiconductor device including the transistor including the oxide semiconductor film, which can perform at high speed, with high yield. A film having a reducing property is formed over the oxide semiconductor film. Next, part of oxygen atoms are transferred from the oxide semiconductor film to the film having a reducing property. Next, an impurity is added to the oxide semiconductor film through the film having a reducing property and then, the film having a reducing property is removed, so that a low-resistance region is formed in the oxide semiconductor film.