In a method for manufacturing a transistor including an oxide semiconductor layer, a gate electrode is formed and then an aluminum oxide film, a silicon oxide film, and the oxide semiconductor film are successively formed in an in-line apparatus without being exposed to the air and are subjected to heating and oxygen adding treatment in the in-line apparatus. Then, the transistor is covered with another aluminum oxide film and is subjected to heat treatment, so that the oxide semiconductor film from which impurities including hydrogen atoms are removed and including a region containing oxygen at an amount exceeding that in the stoichiometric composition ratio. The transistor including the oxide semiconductor film is a transistor having high reliability in which the amount of change in threshold voltage of the transistor by the bias-temperature stress (BT test) can be reduced.