To provide a semiconductor device in which a channel formation region can be thinned without adversely affecting a source region and a drain region through a simple process and a method for manufacturing the semiconductor device. In the method for manufacturing a semiconductor device, a semiconductor film, having a thickness smaller than a height of a projection of a substrate, is formed over a surface of the substrate having the projections; the semiconductor film is etched to have an island shape with a resist used as a mask; the resist is etched to expose a portion of the semiconductor film which covers a top surface of the projection; and the exposed portion of the semiconductor film is etched to be thin, while the adjacent portions of the semiconductor film on both sides of the projection remain covered with the resist.