A method for manufacturing a semiconductor device, comprising: forming a gate electrode layer over a substrate having an insulating surface; stacking a gate insulating layer, a first oxide semiconductor film, a second oxide semiconductor film for a source region and a drain region, and a conductive film over the gate electrode layer; forming a first mask layer over the gate insulating layer, the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film; performing a first etching with the first mask layer to etch the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film so that a first oxide semiconductor layer, a second oxide semiconductor layer, and a conductive layer are formed; forming a second mask layer by ashing the first mask layer; and performing a second etching with the second mask layer to etch the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive layer so that an oxide semiconductor layer having a depression, a source region, a drain region, a source electrode layer, and a drain electrode layer are formed, wherein the first mask layer is formed using a light-exposure mask, wherein the gate insulating layer includes a silicon oxynitride film, wherein each of the first etching and the second etching is dry etching with use of an etching gas including chlorine and oxygen, wherein the oxide semiconductor layer having the depression includes a region with a smaller thickness than a region overlapping with the source region or the drain region; and wherein the second oxide semiconductor film is thinner and has a higher conductivity than the first oxide semiconductor film. forming a gate electrode layer over a substrate having an insulating surface; stacking a gate insulating layer, a first oxide semiconductor film, a second oxide semiconductor film for a source region and a drain region, and a conductive film over the gate electrode layer; forming a first mask layer over the gate insulating layer, the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film; performing a first etching with the first mask layer to etch the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film so that a first oxide semiconductor layer, a second oxide semiconductor layer, and a conductive layer are formed; forming a second mask layer by ashing the first mask layer; and performing a second etching with the second mask layer to etch the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive layer so that an oxide semiconductor layer having a depression, a source region, a drain region, a source electrode layer, and a drain electrode layer are formed, wherein the first mask layer is formed using a light-exposure mask, wherein the gate insulating layer includes a silicon oxynitride film, wherein each of the first etching and the second etching is dry etching with use of an etching gas including chlorine and oxygen, wherein the oxide semiconductor layer having the depression includes a region with a smaller thickness than a region overlapping with the source region or the drain region; and wherein the second oxide semiconductor film is thinner and has a higher conductivity than the first oxide semiconductor film.