An object is to provide a semiconductor device having good electrical characteristics. A gate insulating layer having a hydrogen concentration less than 6×1020 atoms/cm3 and a fluorine concentration greater than or equal to 1×1020 atoms/cm3 is used as a gate insulating layer in contact with an oxide semiconductor layer forming a channel region, so that the amount of hydrogen released from the gate insulating layer can be reduced and diffusion of hydrogen into the oxide semiconductor layer can be prevented. Further, hydrogen present in the oxide semiconductor layer can be eliminated with the use of fluorine; thus, the hydrogen content in the oxide semiconductor layer can be reduced. Consequently, the semiconductor device having good electrical characteristics can be provided.