According to one embodiment, a method of manufacturing a semiconductor device, includes forming a first insulating layer, an oxide semiconductor layer, a second insulating layer, a buffer layer and a metal layer sequentially on a base, forming a patterned resist on the metal layer, etching the buffer layer and the metal layer using the resist as a mask to expose an upper surface of the second insulating layer, reducing a volume of the resist to expose an upper surface along a side surface of the metal layer, etching the metal layer using the resist as a mask, to form a gate electrode and to expose an upper surface of the buffer layer, and carrying out ion implantation on the oxide semiconductor layer using the gate electrode as a mask.