A highly reliable semiconductor device that includes a transistor including an oxide semiconductor is provided. In a manufacturing process of a semiconductor device that includes a bottom-gate transistor including an oxide semiconductor, an insulating film which is in contact with an oxide semiconductor film is subjected to dehydration or dehydrogenation treatment by heat treatment and oxygen doping treatment in this order. The insulating film which is in contact with the oxide semiconductor film refers to a gate insulating film provided under the oxide semiconductor film and an insulating film which is provided over the oxide semiconductor film and functions as a protective insulating film. The gate insulating film and/or the insulating film are/is subjected to dehydration or dehydrogenation treatment by heat treatment and oxygen doping treatment in this order.