According to one embodiment, a method for manufacturing a semiconductor device includes forming a first insulating layer above a polycrystalline silicon semiconductor, forming an oxide semiconductor on the first insulating layer, forming a second insulating layer on the oxide semiconductor, forming contact holes penetrating to the polycrystalline silicon semiconductor in insulating layers including the first insulating layer and the second insulating layer, forming a metal film on the second insulating layer, forming a patterned resist on the metal film, etching the metal film using the resist as a mask, performing ion implantation into the oxide semiconductor without removing the resist, and removing the resist.