A highly responsive semiconductor device in which the subthreshold swing (S value) is small and reduction in on-current is suppressed is manufactured. A semiconductor layer in which a thickness of a source region or a drain region is larger than that of a channel formation region is formed. A semiconductor layer having a concavo-convex shape which is included in the semiconductor device is formed by the steps of forming a first semiconductor layer over a substrate; forming a first insulating layer and a conductive layer over the first semiconductor layer; forming a second insulating layer over a side surface of the conductive layer; forming a second semiconductor layer over the first insulating layer, the conductive layer and the second insulating layer; etching the second semiconductor layer using a resist formed partially as a mask; and performing heat treatment to the first semiconductor layer and the second semiconductor layer.