According to one embodiment, a thin-film transistor and a method of manufacturing the thin-film transistor provided herein achieve enhanced reliability by preventing a disconnection in a gate insulating film at a position corresponding to an end surface of an oxide semiconductor layer. The oxide semiconductor layer includes a channel region, a source region, and a drain region. The channel region is placed between the source region and the drain region. The gate insulating film covers the oxide semiconductor layer in a range from at least a part of an upper surface to an end surface continuous with the upper surface of the oxide semiconductor layer. The oxide semiconductor layer is formed so as to have an oxygen concentration that becomes lower from a top side to a bottom side and the end surface is inclined so as to diverge from the top side to the bottom side.