A semiconductor device having favorable electrical characteristics is provided. The semiconductor device is manufactured by a first step of forming a semiconductor layer containing a metal oxide, a second step of forming a first insulating layer, a third step of forming a first conductive film over the first insulating layer, a fourth step of etching part of the first conductive film to form a first conductive layer, thereby forming a first region over the semiconductor layer that overlaps with the first conductive layer and a second region over the semiconductor layer that does not overlap with the first conductive layer, and a fifth step of performing first treatment on the conductive layer. The first treatment is plasma treatment in an atmosphere including a mixed gas of a first gas containing an oxygen element but not containing a hydrogen element, and a second gas containing a hydrogen element but not containing an oxygen element.