An object is to prevent an impurity such as moisture and oxygen from being mixed into an oxide semiconductor and suppress variation in semiconductor characteristics of a semiconductor device in which an oxide semiconductor is used. Another object is to provide a semiconductor device with high reliability. A gate insulating film provided over a substrate having an insulating surface, a source and a drain electrode which are provided over the gate insulating film, a first oxide semiconductor layer provided over the source electrode and the drain electrode, and a source and a drain region which are provided between the source electrode and the drain electrode and the first oxide semiconductor layer are provided. A barrier film is provided in contact with the first oxide semiconductor layer.