According to one embodiment, a thin film transistor includes an oxide semiconductor layer provided above an insulating substrate and including a channel region between a source region and a drain region, a first insulating film provided in a region on the oxide semiconductor layer, which corresponds to the channel region, a gate electrode provided on the first insulating film, a first protective film provided on the oxide semiconductor layer, the first insulating film and the gate electrode, as an insulating film containing a metal, a second protective film provided on the first protective film and a third protective film provided on the second protective film, as an insulating film containing a metal.