According to one embodiment, a semiconductor device includes a first insulating film, a first semiconductor layer formed of polycrystalline silicon, a second semiconductor layer formed of an oxide semiconductor, a second insulating film, a first gate electrode, a second gate electrode, a third insulating film formed of silicon nitride, and a protection layer. The protection layer is located between the second insulating film and the third insulating film, is opposed to the second semiconductor layer, and is formed of either an aluminum oxide or fluorinated silicon nitride.