The reliability of a transistor including an oxide semiconductor can be improved by suppressing a change in electrical characteristics. A transistor included in a semiconductor device includes a first oxide semiconductor film over a first insulating film, a gate insulating film over the first oxide semiconductor film, a second oxide semiconductor film over the gate insulating film, and a second insulating film over the first oxide semiconductor film and the second oxide semiconductor film. The first oxide semiconductor film includes a channel region in contact with the gate insulating film, a source region in contact with the second insulating film, and a drain region in contact with the second insulating film. The second oxide semiconductor film has a higher carrier density than the first oxide semiconductor film.