In a manufacturing process of a bottom-gate transistor including an oxide semiconductor film, dehydration or dehydrogenation through heat treatment and oxygen doping treatment are performed. A transistor including an oxide semiconductor film subjected to dehydration or dehydrogenation through heat treatment and oxygen doping treatment can be a highly reliable transistor having stable electric characteristics in which the amount of change in threshold voltage of the transistor between before and after the bias-temperature stress (BT) test can be reduced.